The concept of spell copy mechanics has indeed been done many times that I can recall.
This is definitely an intersting attempt but I agree that SoSac could still be abused here. As could silence.
Other than those two, I would like the ability...
The ramp up on cost definitely helps a lot. It would indeed prevent infinite use, but sadly I don't think its severe enough to prevent the above listed spells from being abused.
The problem is that both SoSac and Silence have such a low starting cost (as Zawadx mentions).
... You could always go with an unspecified ramp up. I.e. "skill cost increases significantly after each use". Or something like that. Then just treat sac and silence as exceptions and make it double or triple each time (or more). After a few uses, it would quickly become extremely costly.
