What you're facing is a monoply.
You need a spell. Only one source for it. Man, what I wouldn't give to be in his shoes.
My suggestion depends on just how badly you need the spell upgrade. If your success depends totally upon you having Verdure up to Adept3 level, then you're going to have to cough up the coin. Maybe the guy will cut you a deal but, if he knows he's the only game in town, why would he?
If you can settle for your current level of the spell, then bide your time a bit. Eventually, the guy will get some competition and prices will go down some.