Are you sure?
As far as i remember Roskilde Festival requires the shops to be able to pay some sort of rent. Not sure if it goes for supermarkets aswell though

(Not that Kvickly is cheap..

)
But atleast if it is "normal" prices, i know where the biggest queues will be this year

read somewhere that it will be the exact same prices..
http://ibyen.dk/fokus/roskildefestival/article527277.ecethe man in charge of the supermarket has promised that they wont take extra money for the products, but the same as their regular stores.
I know its a bit more expensive than netto and fakta, but you don't have to spend half a day + money for a train ticket to get food.
And when you stand in the middle of a totally overcrowded Netto, you forget to buy toothpaste, and buy beer instead, know what I mean?
And if you don't have something like a fridge, its kinda hard to have meet for the BBQ...