Ok, I like those Ideas,
I don't have a situation per se', but reading the rules tends to give someone ideas!
Here's how I see magic in my world...
Thorin (our hypothetical caster) uses concentration, gestures, and voice in some combination to mold magical energy (wherever it came from) into a "usable shape" in order to perform a specific task or action. Let us use the example you gave above....
Thorin has focused his "magic energy" into a fireball. He's done this many times. He's also used this ability to "shape" magic energy into fire for other purposes. He's lit campfires, warmed himself and his comrade's on frigid winter days, and even cooked with his fire. We could say Thorin is very adept at controlling magical fire.
One day, Thorin learns of a Mage nearby who is known for using a "Fire Noose" to hold his captives. Those who try to escape the noose only end up with horrible burns. Thorin has use for such a spell... that no-good traitor Garos will certainly suffer for his lack of discretion if Thorin could just master this new technique.
Thorin approaches the mage with the strange new fire spell, and talks him into sharing his technique. But it will cost him!
Exit story:
As I see it, Thorin need only learn a new "shape" and how to produce it. If someone "knowledgeable" were to show him how a few times, then "help" him through the process a few times, Thorin could go off, and practice and perfect his new spell, adding it to his bag of fire "shapes" he's already comfortable with. Given his level of comfort "shaping" fire, I believe he already has the "basics" down pat.
I don't see the learning part as a huge hurdle...
But, Knowledge is a curious thing! It can be given, but it can never be taken back! Once the knowledge is passed on... there is no way short of Murder to have sole possession ever again. Thus, knowledge has a high price... always has.
On the other hand, one purpose of knowledge is to gain wealth, and you cannot gain wealth if you cannot sell your knowledge. Thus, simple economics comes into play. What will the Market bear???
Enter Story:
The Mage may have his special spell, but there is no way Thorin is about to pay him 250 Mithril! Thorin will just have to settle for a nice fireball after all! Garos will pay, but the suffering Thorin was hoping for will not come into play.
Alternately.....
Enter Story:
Thorin barters masterfully with the Mage and both are satisfied with the arrangement. Thorin spends the next few weeks mastering and perfecting his newfound "shape" then goes off in search of his sworn enemy!
Exit Story:
I like ending #2 myself! Although, I'm not opposed to ending #1.
The only problem I have is how to implement Ending #2. Assigning DP or ranks as the price would not affect Thorin's viewpoint in the least.
Joining the Mages Guild, or whatever.... well, easy to say yes, then move on and forget about it!
That pretty much leaves Money, or services rendered, or both.
Wouldn't it be convenient if we had some kind of formula to compute a fair deal with?
Perhaps someone with a little experience in economics could conjure (pardon the pun) up a hypothetical spell market, and create a base formula. There is already a similar system created, and that is the Daily Items. However, in creating these items, the knowledge isn't revealed. Much the same way we can purchase a computer and use it with no knowledge of how it came to be, and no chance of replicating it.
So, to continue with that metaphor.... how much is it worth to know how to build a computer?
We're not talking about microchips... the "magic" exists, we're talking about putting the magic to use.
Hopefully, someone can deduce a reasonable system... it would certainly enhance the game in my opinion!