^+1 on Arch Enemy. Amazing show, tight as hell and great energy. It was quite a while ago, but they had Angela as the vocalist already.
Best concert ever for me was Children of Bodom in 2002(ish?). Front row, club show, epic setlist. Only 15 euro!
Another great show was Dragonforce this year. 17,50 euro. Totally worth it. Great fun and they played very tight actually.
I'm a big Epica fan, a Dutch female fronted metal band, and I was pretty close with them when they first started out. I saw quite a few of their shows and the ticket price was very low. They're still great, the latest album kicks ass. I actually got to sing part of 'cry for the moon' when Simone (the vocalist) put the mike under my nose
I also saw quite a few cool bands on free festivals, which is also great. Except in the case of Anathema, which was one of the most boring performances I've ever seen.