Social costs rather than direct personal costs
In my fantasy novel, a 12 year old boy gains the ability to increase or decrease people's desire to say what they are thinking. This ability isn't telepathy, exactly, but it's similarly powerful and has a somewhat similar effect, so I thought I'd explain what I'm doing, and why I have no direct costs for using the magic. I do have limitations, indirect costs, and a big one-time cost for gaining magical ability.
The limitations are: First, a mage can influence only one person at a time. Second, use of the magic requires the mage's full attention, which leaves the mage vulnerable for the duration. Third, the use of magic is visible to other mages in the area, so you can't influence people in secret. (This third limitation matters only later in the story; at first there are no other mages to observe the boy's use of magic.)
The main reason I don't want direct costs for using magic is that I'm deliberately setting up a "tragedy of the commons" problem for a later book, after magic has become widespread: There's a large but finite pool of stuff that fuels the magic; each use consumes a tiny but non-zero amount of the magical stuff; there's a way to replenish the stuff, but it's onerous; and there's no direct cost for using the magical stuff. So though each use has no direct cost for the user, it does have a cost to the community in that it diminishes the pool of available magic. In the later book want to explore how society quakes when the stock of magical stuff runs low, the cost of use is low, and the cost of replenishing is onerous.
More importantly for the first book: The social costs of of having this ability are high enough to cause enormous problems. We all have secrets and other thoughts that we very much want to keep private. If you knew that this kid could make you say what you're thinking, then when you're near him you're probably more likely to think of the things you're afraid he'll make you say. People ain't gonna like having this kid around. In the first book, it's these social costs that I want to explore.
There is one big cost (or at least a risk) associated with gaining magical ability in the first place. To gain magic, you must have a near-death experience in a particular place (the source of the magical stuff). The "near" part of "near-death" is problematic. There's significant chance that you'll end up not only nearly dead, but really most sincerely dead. But once you've survived that, the mage pays no direct cost for each use.
Given all of that, I think I'm okay making the use of magic "free."
Dale