Where I live we have a prom at the end of year 9 and then at the end of year 13, and my year 9 prom dress was around £175. I was going to get a £300+ dress, but then I found the dress I got that was half the price and almost completely identical ao I got that instead.
But there were people at my prom who spent double/triple what I did, and people spent a lot less than I did. It really just depends on what your budget is and how much you like the dress. You might get and opportunity to wear it again, or it might sit in the back of your wardrobe for years, but you'll be wearing it at prom and you want to make that night as special as possible, including your perfect dress, whether it cost £15 or £300. Wear what you want, what you're comfortable with buying.