I've been having the same problem for a couple weeks now. My online money seems to be stuck at 86k in the bank and 10k in cash on me. Every time I load up online, that's how much money I have, no matter how much I earned or spent last time I played. I don't get it, everything else seems to be saving now: my rank, my awards, etc. So it's great for spending that much money on things and having it all back, but not good if I actually want to save any money. I'm hoping with the stimulus package I won't get screwed out of my money. Maybe I'll even get lucky and my game will be forever stuck at 500k instead.
This all happened the first time I bought an apartment. I was winning races and bets and got up to 87k. I bought the 80k apartment with a garage for the achievement and logged off after that. I came back and still had my apartment, but also still had my money. Ever since then, my money has been stuck where it's all. Went through and robbed all of the stores, and all of that money went to waste. Did a couple missions, that money went to waste. Spent money on haircuts for the platinum award and got all that money back though. I guess I should buy the outfits and private dances and mods so I can get those awards while I seem to have unlimited money.