People with significant assets agonize over whether to pay off their mortgage or keep money invested, but the decision involves dozens of interacting variables (capital gains taxes, mortgage interest deduction, expected returns, refinance scenarios, opportunity cost on cash) that are nearly impossible to reason about in your head or a basic spreadsheet.
A web app where users input their mortgage details, brokerage holdings (with cost basis), tax bracket, and state. It runs Monte Carlo simulations showing probability-weighted outcomes for pay-off vs. invest vs. hybrid strategies over 5-30 year horizons, accounting for capital gains tax drag, lost mortgage interest deduction, refinance trigger points, and sequence-of-returns risk. Outputs a clear recommendation with confidence interval.
Freemium — free basic comparison, $29 one-time for full Monte Carlo analysis with tax optimization; $99/year pro tier with refinance rate alerts and annual rebalancing recommendations. Affiliate revenue from mortgage refinance and financial advisor referrals.
This is a real, high-stakes decision (often $100K-$500K+ at play), and the Reddit thread proves people agonize over it. However, it's a one-time or infrequent decision, not a daily pain. People suffer intensely but briefly. Score docked because many ultimately just pick one and move on without a tool — it's painful but not blocking.
TAM is narrower than it first appears. You need: homeowner + mortgage above 5% + $500K+ brokerage + financially literate enough to care about tax drag but not so sophisticated they model it themselves. In the US, roughly 4-6 million households fit this profile. At $29-$99, ceiling is maybe $50-100M TAM. Decent for a lifestyle business, too small for VC.
The target audience is wealthy and the decision involves enormous sums — $29 is a rounding error on a $500K decision. But free calculators exist (even if inferior), and personal finance users are notoriously cheap. The Reddit thread shows people asking strangers for free advice rather than paying $200 for a CFP. $29 one-time is priced right, but conversion from free will require very strong trust signals. The $99/year tier will be hard — ongoing value is thin for a one-time decision.
A solo dev can absolutely build an MVP in 4-8 weeks. Monte Carlo simulation is well-understood, tax bracket math is deterministic, and mortgage amortization is textbook. The hard parts are: (1) getting state-specific tax rules right for all 50 states, (2) cost basis / tax lot modeling for realistic liquidation scenarios, (3) making the output understandable to non-quants. None are unsolvable, but state tax accuracy could eat weeks. Ship with 10 states first.
This is the strongest signal. No one owns this specific decision as a product. NerdWallet/Bankrate are too simple. ProjectionLab/Boldin are too complex and general-purpose. Financial advisors are too expensive. There is a clear gap for a focused, tax-aware, Monte-Carlo-powered tool that answers exactly one question well. The wedge is real.
This is the biggest weakness. The core decision is made once — pay off or invest. After that, there's no reason to come back daily. Refinance rate alerts and annual rebalancing are nice but not compelling enough for $99/year. Most users will churn after getting their answer. This is naturally a one-time purchase product, not a subscription. The affiliate revenue from refinance referrals could be meaningful but is dependent on rate environment.
- +Clear, validated pain point with strong emotional engagement (468 upvotes, 339 comments on a single thread — and this question appears weekly across r/personalfinance, r/fatFIRE, r/Bogleheads)
- +Wide-open competitive gap — no one owns this specific decision tool despite enormous search volume for 'pay off mortgage or invest'
- +Target audience is wealthy, which means high affiliate revenue potential per user (mortgage refinance leads pay $50-$200 per qualified lead)
- +Low technical risk — the math is well-understood and an MVP is buildable in weeks, not months
- +Natural SEO/content marketing wedge — every personal finance article about this topic could link to this tool
- !One-time decision product: user gets their answer and never returns, making the $99/year subscription tier very hard to sustain — the real business model is likely one-time purchase + affiliate, not SaaS
- !NerdWallet or Bankrate could build this as a feature in a weekend and bury you in SEO — you're competing for the same keywords against domains with 90+ domain authority
- !Accuracy liability: if your Monte Carlo simulation leads someone to make a $300K decision and they lose money, you face both reputational and potential legal risk — disclaimers must be ironclad
- !The $99/year pro tier may not retain: refinance alerts and annual rebalancing are weak hooks that don't justify recurring spend for most users
- !Cost basis and state tax modeling for 50 states is a long tail of edge cases that could consume disproportionate dev time relative to the audience who needs it
Free online calculator that compares paying off mortgage early vs. investing the difference, showing total interest saved and projected investment growth side by side.
Similar to NerdWallet — a simple deterministic calculator comparing fixed mortgage rate against an assumed flat investment return rate.
Comprehensive financial planning simulator with Monte Carlo simulations, allowing users to model retirement scenarios including mortgage payoff strategies within a broader financial plan.
Detailed retirement planning platform that includes mortgage payoff scenario modeling as part of a holistic financial plan with Monte Carlo simulations.
Robo-advisors offer tax-loss harvesting and some scenario planning, and traditional advisors run similar analyses using proprietary tools
Week 1-2: Build core comparison engine — mortgage amortization schedule vs. investment growth with simple tax-adjusted returns (federal only, top 5 states). Week 3-4: Add Monte Carlo simulation layer (1,000 runs using historical return distributions) with clear visualization showing probability ranges. Week 5-6: Add cost basis input for one brokerage account, capital gains tax calculation on liquidation, and mortgage interest deduction impact. Ship the free tier (deterministic comparison) and $29 tier (Monte Carlo + tax). Skip refinance alerts, skip annual rebalancing, skip the $99 tier entirely at launch. Validate willingness to pay before building recurring features.
Free basic calculator (captures SEO traffic, builds trust) -> $29 one-time for full Monte Carlo + tax analysis (core revenue) -> Affiliate revenue from mortgage refinance leads when the model identifies refinance as optimal ($50-$200 per lead, potentially larger than direct revenue) -> Financial advisor referral partnerships for complex cases -> Only add $99/year tier IF you find a retention hook that actually works post-launch. Do NOT assume subscription will work — plan the business around one-time + affiliate.
6-10 weeks to first dollar. 4-6 weeks to build and ship MVP, 2-4 weeks to get initial organic traffic from Reddit/HackerNews launches and early SEO. First $29 purchases likely within week 1-2 of launch if you seed it in relevant Reddit threads (r/personalfinance, r/fatFIRE, r/Bogleheads). First $1K month likely 3-4 months post-launch. Affiliate revenue takes longer — 4-6 months to set up partnerships and build enough traffic to generate meaningful lead volume.
- “I'm looking for some perspective on whether I should pay off my home or keep my funds invested”
- “What variables am I missing?”
- “How much taxes are you gonna owe? I don't see how paying 15-20% capital gains tax rate is better than paying 6%”
- “the gap isn't as big as you think — you'll save 6% but you'll give up the return... currently 3-4%”
- “you lose your mortgage interest paid deduction on your tax return”
- “you should probably talk to a financial planner”