Well I was looking at going a few days early and leaving monday.
Easy jet, departure £29, return £23. You don't need extra luggage, you can fit more than enough in a bag that'll fit in the overhead compartment thing. Otherwise, one bit of hold luggage is £18. £9 booking fee. If you're going alone that'll be a problem, if you have any friends you can book together and split the cost. Take 1 hand luggage and you can share a large suitcase between you for whatever else you need, but I really don't see why you'd need hold luggage unless you're going on a proper holiday or taking your shoe collection or something.
Even if you're going by yourself and have to pay all the fees, it'll only be £79.
This is all departing from Gatwick BTW.
e: Oh yeah, I'd rather go a day early and spend the £40 I'll save on flights on 1 night at a hostel. Assuming you don't mind a 10 person dorm it's only £15. But then again, maybe I'm unique in not wanting to go just for the festival. The way I see it, once you've gone all the way there just to see a festival you might as well spend a few extra days doing something else.