How much does a fund's expense ratio really cost me over 30 years?

A lot more than the small percentage suggests, because the fee compounds against you every year. The expense ratio is the annual slice the fund keeps — 0.03% means $3 per $10,000, while 1.00% means $100. That gap looks tiny but it drags on your whole balance for decades. On a $100,000 portfolio growing 7% a year for 30 years, the difference between a 0.05% fund and a 1.00% fund is roughly $150,000 in lost ending value, because every dollar skimmed never compounds. This is why low-cost index funds (often 0.03%–0.10%) are so powerful: you keep almost all of the market's return. When comparing funds, the expense ratio is one of the few predictors of long-run performance you actually control. Try wealthserene.com/tools/opportunity-cost to see the drag on your own numbers.