So Ingolf, the Mage in the Water Monestary who sells you stuff (and shrunk you the rat bastard
) has an infinite supply of blank spell runes due to the fact he interacts with the rune maker. Every time he uses it he gains a new one in his inventory if he has none (actually, he gets two I don't know why). These can then be traded into the City water mages for 250 xp a pop.
The runes themselves cost a laughable 50 gp per item. Net cost of 45 as you get 5 papers you can just sell right back if you want.
Only problem is the mage takes ALL of the runes of the same type. Normally this isn't an issue as you just drop the extras and give them one at a time. Blank runes are stackable so you'll have to put the extras in a nearby container. The nearby bookcase works well for this.
Honestly, it's not really worth it. By this point you should be God amongst mortals, and it's a bit tedious. But it's there if you're so inclined :D