Not giving an exact figure here but the amount of funds on Britcoin are usually 100x the volume of the market.
But it varies: there was a point where we were getting 3k volume per day and had 10k GBP, 10k BTC.
Usual MtGox volume is 20k, so I'd say they likely have ~200k online.
I think you are off by at least an order of magnitude, or two.
The recent volume on MtGox has been over $200K USD per day.
100x that would give an estimate of $20M USD in the market on Mt Gox.
(I would guess this is high, 10x = $2M USD seems more reasonable at this time)