Week 6 Day 2: The $100/Month Investor

$100 a month is achievable for most working adults. Over 40 years, you put in $48,000. The market hands you $216,000 on top. For every dollar you invested, compounding added $4.50. That is the return on patience.

Let us scale this up to see the possibilities. At $200/month for 40 years at 7%: $528,000 (you put in $96,000). At $500/month: $1,320,000 (you put in $240,000). At $1,000/month: $2,640,000 (you put in $480,000). The pattern: at 40 years, your total contributions are roughly 18% of the final balance. The market provided the other 82%. This is the ultimate argument for starting early -- not because the early contributions are large, but because they benefit from the most compounding cycles. Your first $100 contribution at age 25 compounds for 40 years. Your last contribution at age 64 compounds for 1 year. That first $100 becomes roughly $1,500. The last $100 becomes $107. Same effort, 14x different outcome, purely based on time.

The mathematical proof of why early contributions dominate is found in the future value of an annuity formula: FV = PMT * [((1+r)^n - 1) / r]. The key variable is n (time). But what is less obvious is the derivative with respect to n: dFV/dn = PMT * (1+r)^n * ln(1+r) / r. This derivative increases exponentially with n, meaning each additional year of investing contributes more to final wealth than the previous year. Conversely, each year of delay costs more than the previous year of delay. For a $500/month investor at 7%, delaying from age 25 to 26 costs roughly $38,000 in terminal wealth at 65. Delaying from 26 to 27 costs roughly $41,000. Delaying from 34 to 35 costs roughly $67,000. The cost of delay accelerates -- it is not a constant penalty but a compounding one.