For an S&P 500 index fund, many come with no minimum investment. For an S&P 500 ETF, you might need to pay the full price of a single share, which is generally upwards of $100—but some robo-advisors like Stash offer fractional shares for as little as $5.
If you’re investing in individual stocks, you’ll just need to pay the cost of the share, which varies by company—you’ll find some for under $100 and others for $350+.