I've made a small script to help you understand how a transaction (buying/selling usd/btc) affects the market at MtGox. It gives you the answer to question like
- I want to buy X Bitcoins, how much $ does it cost?
- I want to spent X $, how much Bitcoins do I get?
- I want to sell X Bitcoins, how much $ do I get?
- I want to get X $, how much Bitcoins do I have to sell?
and also calculates the bid/ask price that would be in effect after you'd perform these trades.
This was very helpful for me (I wanted to know how many BTC I would have to sell in order to cover all my expenses so far) and I hope it is for you, too