Ideas for vanity prizes:
Lowest Winning Score, as you suggested (always amusing to lose/win something like 2 points to 1)
Biggest Winning Score (obviously)
Domination (all 8 provinces bought, first player only, kind of link 147 break in snooker)
Quickest Win
Most Cursed Player (will involve running total though, either tot up at end, or keep running score of curses given that count even if trashed)
erm...
Quickest Province (bit luck based though)
Biggest Purchase, as you suggested is good though it has to be coin actually spent of course
hmmm, drawing blanks now.