The next time you read a fantasy novel, consider how (and if) the author deals with the costs of doing magic.

Probably the most common way this is handled is to have magic use up the magic-worker’s physical and/or mental energy. After doing spells, the magician is tired and hungry. After doing long, hard and complex spells, she may sleep for days, have reaction head-aches and so on.

One of the more interesting variations of the idea of energy-use is found in the Frost Series (Demon Blade and Frost) by Mark A. Garland & Charles G. McGraw. In these books, all magicians are fat, because working magic literally melts the fat from your bones and the fatter you are when you start, the more magic you can do before you run out. Men are generally more powerful magicians than women simply because they have bigger frames to place the needed fat on.
(If you’d like to read either of these books, Demon Blade is out of print, but can be found in E-book form here in the Baen Free Library. Frost is available in paperback at Amazon.)

In Stranger at the Wedding by Barbara Hambly, using magic takes some energy and if you do a lot you get tired, but the real costs are social. Wizards are open to censure and prosecution (including burning!) unless they join the Wizard’s Council, but the Council has stringent rules of its own that wizards must follow.

Other possible costs that may or may not have actually been used in fantasy novels:

• The ingredients and paraphernalia of magic are so expensive that not many people can afford them.
• Spells are so complicated that it’s easy to make mistakes and have spells backfire (cost one), so it takes years and years of study to be proficient and not dangerous to yourself (cost two).