Don't spend an amount you'll regret! Disregard what the others are boasting about. Some have parents that can afford the Earth, some don't. Have a good look online at photos of prom dresses and consider whether you can alter a second hand dress.
A department store prom dress could set you back $250 or a lot more, and you'll see the same dress on others, perhaps. An elegant line, a colour that really suits you, those are what count, and good draping material rather than glitter and glitz. Be yourself and go for what you know suits you.