Making Magic Work: Cost of Magic
One challenge in fantasy writing is the craft of designing a magic system that makes sense - that's believable for the reader - but which is flexible enough that you can continue to toss out new curves over the course of a series. Often, writers will fall back on some set of tropes that they're familiar with. The opening to Eragon, for instance, reads a lot like a write-up of a Dungeons and Dragons game. The writer seemed to quickly realize that wasn't going to work for the long haul, however, and crafts a magic system which is fairly unique for the rest of the books.
Jim Butcher's Dresden novels have their own fairly unique tropes. Wizards can't be near technological devices without messing them up, for instance. Wizards have a pool of inner power which they can use up, and replenish by resting. Wizards have varying levels of power. But as the series rolls on, we see those tropes shift somewhat - we learn, for instance, that the "tech restraint" was true even hundreds of years ago - that magic interferes with different things in different times, but always has "issues" with something common to the non-wizard. We learn that stimulants can temporarily boost magical power (at some cost!). And Butcher gives us a much better idea, as the series goes along, what that variation in power level is like.
Getting to classics, Tolkien hardly uses wizardry at all. Actual spell casting is something he brings to the fore only rarely in his Middle Earth books. But when he does, he focuses on one particular element of magic common to almost all excellent fantasy: cost. In his case, there are deep veins regarding the price of power and the corruption power can bring with it, using the Rings of Power as the tool to discuss the issue.
Orson Scott Card's Hart's Hope creates a world where magic is created by killing something. The larger the creature, stronger its life force, and closer to the magician the being is, the more magic you derive from the killing. And Card takes that thinking to its logical and fairly frightening conclusion.
It seems obvious on the surface: magic must have limits. If it didn't, wizards would use magic for everything, and writers would have a hard time crafting interesting stories about someone who had so much power with no cost. It's possible, of course to create stories where magic has no cost - you CAN create a fun story about magic without price. But at that point you need some other limit instead. Someone who could only create a 10-watt light in their hand could have no cost involved in the process and probably not unbalance a story. Someone who can create any size light of any intensity should probably have a price involved which grows larger as the light gets bigger and/or brighter.
In the Ryan Blackwell novels, I've fallen back on the familiar trope of magic wearing out the spellcaster. Use magic too much, and you won't have anything left for later. Ryan, however, has a couple of twists. His story takes place on top of a huge ley line nexus. This gives him a well from which he can draw extra magic - at some risk (cost!). That same well has a tendency to attract dangerous elements of the magical world, though (cost!). And in fact, as Ryan learns, the more magic one uses, the more likely one is to attract the attention of certain parties who move against powerful new magicians. Sometimes just to eliminate future rivals. Sometimes as food. Sometimes a little of both.
That's not really a cost, exactly, but it definitely creates a "threat" around the use of magic!
In the end, part of what will make a magical story memorable and interesting will be the way you set up the magical system, and the unique/creative elements you invent for your magical milieu. The less you rely on "standard tropes", or at least create strong twists on those tropes, the more unique the magical element of your story will become. Creating interesting costs can be a challenge, but having some twist which differentiates your story from others will lend it strength, interest, and memorability.