I tried to account for the 0.65% fee per trade and apparently failed to do so, as the fees have absorbed just about all my gains.
The way fees are applied seems to confuse a lot of people.
They are also a bit high.
As far as I understand you get:
a 0.65% commission on the price you ask when you sell
a 0.65% commission on the price you bid AND on the coins you get when you buy.
0.65% of the BTC amount when you buy
0.65% of the USD amount when you sell
Here are the notifications I received by email:
You Sold 10 BTC for 18.89!
You Bought 5 BTC for 18.511!
These are the actual trades that occurred, they do not include fees.
In the first trade you sell 10 BTC for 18.89, so you get 188.9 USD before fees. You pay 0.65% of the amount you get (1.227 USD) and keep the rest (187.673 USD).
In the second trade you buy 5 BTC for 18.511, so you get... 5 BTC before fees. You pay 0.65% of the amount you get (0.032 BTC) and keep the rest (4.968 BTC).
and here is the corresponding Mt Gox account history
Sold BTC 10 for 18.7673 -10 187.673 30.718 925.288
Bought BTC 4.97 for 18.6302 4.968 -92.555 40.718 737.615
Okay, this IS confusing, anyway if you look at the third and fourth column, it's the same as in my calculations above.
For the first trade, you give 10 BTC (third column) and get 187.673 USD (fourth column), which is 188.9 USD minus 1.227 USD fee.
For the second trade you get 4.968 BTC (third column), which is 5 BTC minus 0.032 BTC fee, and you give 92.555 USD (fourth column), which is 5 BTC times 18.511 (the original rate).
Anyway, you can just forget this last confusing part, since after the relaunch the history page was redone, and now shows trades and fees separately.