I find the EUR30/day perfectly adequate.
I stay almost always in albergues, I like to make my own little lunch, and I often cook my own dinner, or together with others. I find it charming to walk to the local shop and buy things I do not see at home, and prepare a dinner for myself or together with others. That should not set you back more than 5 Euros/day for food.
But I find the pilgrim menus most often are good, at least in the meaning "enough" (two dishes + wine, normally), so I sometimes try them. The only thing is that most often, they are served with fried potato chips: Not my favorite over several days...
As others mention, prepare for other costs, f.ex. at the farmacia: Compeed is expensive, and I use it now and then as prevention for growing serious foot trouble.
In daytime, when hot, I like to air my feet joined by a cold beer (or 2), and I like Spanish red for lunch/dinner (in line wth Stephen there 8) ). This may lift the daily cost a bit, so I agree on 30/day :wink:
But I have had many days on 15, and a few on 40-50 Euros.
Edited: As I like to cook, like many others, IMHO, the best experience is to team up with other pilgrims, go together to the shop and find something all like, and then cook and do the table (and cleaning...) together. Some of my best experiences have been the cameraderie of fellow pilgrims around such tables. And it tends to come down to 3-5 Euros/person...