You can bring whatever you want to the camp. A lot of people BBQ. I've even got away with bringing sandwiches into the main stage area, which is a good way to avoid spending money on the tempting vendor stands.
The festival stand prices seem pretty average (3-7 euros for anything substantial), but you can find some cheaper options at the village biergartens too. I think beer was 3 euros, but you could buy cans at the onsite market (crawling distance to the main festival biergarten) for half that, to save money if you you didn't carry any.
yeah, wacken is no exception
the food is also overpriced.
But it is allowed to bring your own food, and there are some supermarket stands at the festival itsself. Those are still quite pricey (compared to normal prices) but already way cheaper than the food and drinks prepared on the fest.
You can also go to the supermarket in the village, which also has higher prices than normal, but also still cheaper than on the festival grounds.
Do keep in mind that the supermarket is always flooded with metalheads, and it can be quite a walk