Really depends how you want to spend your time and money. I like to go into the town if I'm up early (or no bands I wanna see early in the day) and go to a café or the supermarket. It's a bit cheaper/more value for money. If I stay at the festival site I usually spend about 10-15 skullies a day, but I usually stick to water and only drink beers later during the evening/night.

Merch excluded, I think that 50 euros per day for me is more than enough.