Somehow I have managed to get by on $150 or less, every single Fest I've attended (this will be my 5th).
I've avoided buying merch/records for the most part and basically just spent it on food/gas/etc. I bring extra/excess to pay for the hotel/car rental.
Refraining from merch and records is the hardest but its the only way I can control my spending.
If I brought ANYTHING more than $150 for spending I would have too many heavy bags to bring home with me full of crap, and it'd be all gone by the time I got home.
Oh, and the fact that I don't drink alcohol and therefore spend nothing on beer saves a lot
So I ended up having a little more than usual this year ($300 total; $100 cash, $200 on a gift/credit card thing I got as payment from doing a focus group) and therefore splurged a little bit more than I normally would. Bought 5 records and a Vaya Bags purse at the Fest registration flea market. Everything else went to food and whatnot, and I come home with about $60 of it left in total. Splurged because I had the option to this year.
I actually almost did mail my records back home to risk them not being damaged on the plane back, but decided to save money and packed them extra tightly in my suitcase (in a vinyl mailer). All came back in one piece.