The price displayed on the cryptocurrency’s Detail page is the mark price, which is the midpoint of the bid and ask prices. When you’re buying a coin using a market order, your order may execute at the ask price, which is higher than the mark price.
When you’re selling a coin using a market order, your order may execute at the bid price, which is lower than the mark price.
Robinhood doesn’t charge commission fees. Any price difference you may see between the estimated buy/sell price and the execution price is due to market movement.