With the bonus program, you can cut the number by a large amount. Also, buy in bulk when GREE has their 40% off sale. Lastly, buy iTunes cards when they go on sale. Best Buy has them at 20% off relatively frequently.
Your $10k figure is less than $200/member of a 50 member guild. With all the savings I described along with the bonus program, it's much less than that.
Spending $100 every couple of weeks on something that entertains you isn't that big of a deal to many people. I spent about $90 last night taking my family to dinner (which wasn't even all that great), and I wouldn't consider myself rich. Surely there are a few people who are rich that play this game that could easily spend much more.
I don't think it's too far fetched. I spent a few bucks a month on this game before the hacker guilds became rampant. Now I spend considerably less, but I can see how it is conceivable.