I'd have to say the most money I've spent on one MMO would have to be WoW.
1) Collector's Edition of WoW
2) TBC, WoTLK, and Cataclysm expansions
3) 4 years or so of subscriptions
4) Paying to server transfer 3 characters at $25 a pop.
It makes my lifetime sub to TSW look like an absolute bargain by comparison.