Price is a big factor when choosing an investment app, so it's crucial you know what you're paying for. After all, any costs or fees you aren't aware of can eat away at your investment earnings over time.Many investment apps are free to download and offer zero-commission trades, which means that investors aren't charged for the broker executing the trade.Investing in mutual funds or ETFs, however, will likely require paying management fees or expense ratios.According to Investopedia, investors should expect a reasonable expense ratio for an actively managed portfolio to be between 0.5% to 0.75%. For passive or index funds, the typical ratio is lower at about 0.2% and can be as low as 0.02% or even less in some cases. These are benchmarks that you can use when shopping around for an investment app.