Low-cost index funds are among the most advantageous investment vehicles for those focused on the long term. It's important to know a fund's expense ratio, which denotes how much money in management fees you'll pay, before investing your hard-earned dollars. Here are some top low-cost index funds and their expense ratios:Vanguard S&P 500 ETF 0.03%Vanguard Large-Cap ETF 0.04%Schwab U.S. Large-Cap ETF 0.03%Vanguard Mid-Cap ETF 0.04%Schwab U.S. Mid-Cap ETF 0.04%Vanguard Small-Cap ETF 0.05%iShares Core S&P Small-Cap ETF 0.06%Schwab U.S. Broad Market 0.03%iShares Core S&P Total US Stock Market 0.03%Vanguard Total Stock Market 0.04%