I agree with Mike. Possibly the best places for Crumble Undead would be in the Varrock Sewers, Draynor Manor (upstairs where ghosts are). If there are any better places, correct me.
You should only train magic with the number of runes you can possibly afford. I paid probably 350K getting 62 magic-65 magic and I was on a tight budget back then.
Other than that, either use Crumble Undead or bolt spells. They're both pretty alright training. For bolt training, places like the popular Wizards Tower (with the caged Lesser Demon) are alright, and so are places like the Hobgoblins in the peninsula near the Crafting Guild and the other places mentioned.
Bleh, I can't think of many more places, my brain is tired from school exams.