I never bothered with the prom just because in theory, it was just a bad idea to go. It felt like a waste of time and money.
£5-£10 roughly for tickets.
£100+ to rent a good suit.
£5 for meal.
£10 to cover anything else needed, like petrol to get me out of the dump.
On a good day, it may be £105+. What do I get with that? I get to spend hours surrounded by people I really don't like for a large multitude of reasons, drinking non-alcoholic drinks and listening to music that I wouldn't even listen to ten seconds of if someone linked me a Youtube video of it. For at most £24, I can get a 24 pack of beer, more than enough to get me drunk. I can spend it at home doing whatever I wish only talking to people I like and listening to music I do like. You could make the case "but it's a unique experience!", but so is being run over by a car. I really wouldn't recommend it.
My sixth form will be doing a similar thing (a leaver's do), £10 for tickets and it's likely formal wear. Which, as well as the prom, will be filled with people I don't particularly like. The only difference is I can get drunk, but it'll still be filled with people I don't like in a loud atmosphere filled with bad music. So I'll most likely give it a miss.