Here's the thing... (again...)
Like Martin says, Roskilde donates all profit to charity. There's no question about it, since the Danish tax administration ("SKAT") makes sure they do - and have made sure since around 1972.
The other thing, every single time they choose something to donate to, they'll be under insane scrutiny. Once again, the tax administration has to sanction their choice; Danish politicians will criticize them if they donate to the "wrong" things. This donation is political (pretty much whenever they donate to NGOs), that donation is corruption (like when they donate football fields to the city too close to when the city council decides to do something beneficial to the festival).
Funny side note while we're on the subject: Kim Larsen has famously not appeared at the festival since 1974. Last time I heard him asked about it in an interview, he claimed that this was because at some point around 1973 or 1974 he and the rest of Gasolin' asked where the money went - and the answer was that it mainly went to building a youth community house in Roskilde. So, supposedly Gasolin' showed up at the opening of that place, and they saw some "drunk kids". "And that wasn't something we wanted to support". Sure, but they were fine with supporting the festival in 1971, the one year when it was pure profit for the arranger, Carl Fischer. And Kim is still fine playing at every for-profit festival or event in Denmark (as long as they cover up the posters advertising the sponsors that pay his wages...) But Roskilde
has, according to him, "become all about profit".
Which is supposedly why he and Gasolin' asked for "$20,000" in 1976 (literally, dollars, according to both Leif Skov and Kim himself), and the festival said, "no thanks". Nothing at all to do with him seeing Dr. Hook getting that exact amount and feeling he should be paid like a superstar. :p
Nah, the one thing that's bullshit is people making "it's become all about profit" their go-to excuse for claiming "everything's going down hill" - or even for doing stupid shit.
To add to Martin's post, in 2013 the year's donations amounted to 35 mio. DKK.