After casting a spell 100 times (or some number you decide inversely proportional to cost?), you can cast it with a 20% discount as long as it remains unchanged. When casting it for the 100th time, you are greeted with a chat message saying "you have mastered <spell 5>", (whatever you named it). This streak would reset when you change it in any way.

Or perhaps it may be easier to have this only work for those tomes you find, that way you dont need to worry about keeping track of all ten spells individually and whether they have been changed or not, plus with certain combinations it may be unbalanced?
 
I know the ring exists but I feel like spell mastery of some kind would be a cool addition.