It seems like if the magic is widespread (everybody has it, or could have it if they wanted to learn), it doesn’t need to have a large cost, or any cost–cost exists partly as an equalizer and if everyone’s paying the same amount, they might as well be paying nothing.
On the other hand, I’ve heard people complain that the widespread magic in Harry Potter has no cost. On the other other hand, if I were to poke at the worldbuilding there, that wouldn’t be where I would start, so I could discount those opinions as “different priorities/tastes”. (Back to the second hand, the complaint that it has no limits is more valid, but still doesn’t bother me.)
Internal versus external. In fantasy the expectation is that the cost is internal. You can’t throw any more fireballs today because it will drive you insane, corrupt you, kill you, or kill someone you love.
In SF, you don’t hear “Yes, Captain, I could give you ten more minutes at warp 9.8, but I’m down seven fingers already and I don’t want to lose my thumbs, too.” The cost of technology is external–money, or time, or effort. It can kill you, but not as a normal part of the routine.
Which should mean that if magic is treated as a science in your world, it doesn’t need an internal cost. To the people in that world, it’s not magic, it’s another branch of science.
(Does not apply to magic acquired as a gift from the gods, demon summoning, etc.)
Low- or no-cost magic is not the same thing as magic with no limits or no rules. Just because throwing another fireball won’t make your leg fall off doesn’t mean you have enough ACME Fireball Starter to do it (you have to buy more, so there’s your external cost), or have studied enough to pass the exam so that you have permission to learn the spell, or that fireballs are even possible under the laws of your universe.
This post was inspired by my thoughts on (part of) David B. Coe’s Thoughts on creating magic systems from a while back, where he says he tries to make sure his magic has internal consistency, limits, and cost. I agree on the first two. (And without internal consistency, you couldn’t treat magic as a science.)