One of my favorite descriptions of magic is in Jonathan Strange and Mr. Norrell, by Susannah Clarke. It's deliberately vague, we rarely see a description of what magicians are actually [i]doing[/i] to make magic happen. They talk a lot about formulas, and mention techniques by name without describing them. It makes for a fun, arcane "technobabble" that I personally really enjoy. 

"No, see here? You've forgotten to set a boundary parameter. If you cast a spell like this, it'll keep going until it burns up all its fuel, or runs out of targets, whichever comes first. Try [i]Cervantes' Discriminación[/I], it's a great framework for selective description." 

I also like magic that has a cost, I think it would just be material components, that are usually burned up in the process. The required components are symbolic, and depend on the nature of the spell. DnD has great examples, it could be something like a quantity of fine sand, special herbs, maybe blood, bone or hair(though maybe these are seen as kind of sloppy and uncouth). 

This is something I don't want to put too much specific thought into, though, as it's really just a background for social roleplay.