Should I go to prom?

I don’t know if I want to go to prom or not. It’s four months away. I can’t really find a dress that I like. My best friend (who nearly died) is begging me to go; I feel like the only reason I’ll be going (if I do go) is for her. I’m tempted not to go because it will save me a lot of money, and I think it sounds kind of boring. My other friends are asking me about transport, but I don’t even have an answer about whether I’m going or not. I’ve made a list of pros and cons, and the cons outweigh the pros. also, I’d like to go to a music festival this year, so I’d like the money for that; if I went to prom I wouldn’t be able to afford the festival tickets. It just doesn’t sound like my thing, and I think it sounds boring. But I’m scared I’ll regret it if I don’t go; plus my friends wouldn’t understand why I don’t want to go, and I’d have let my ill best friend down. So what do I do?