Remember to take into account the setup, tape costs and mixdown time.
The first few times I went into a studio I was surprised that the drums could take a couple of hours to setup and get rid of squeaks and rattles.
Then getting the levels right & mic placement etc.. we were usually taking several hours to set everything up properly
If you want to keep your session, you have to buy the tape too... another thing that surprised me since I figured it was included in the cost.
If you want a pro mix you'll need to get it mastered as well.