Affecting stock when multiple units can be on one order

Here is my scenario:

In my cart, I have seats available for a class. Let's say the cost of the first seat is $750, and every seat after that costs $550 (on the same order). So if one person buys 3 seats, the cost will be $750 + $550 + $550. I know I can create this structure using attributes etc.

However, the max number of seats I can sell is 40. If one order equalled on seat, then this would be easy... I could set my stock amount as 40. But what do I do since one order could have any number of seats?

How do I set it so that, for instance, one order with 4 seats brings the stock amount to 36 (40 minus 4)?

Thanks guys.

Anything guys?

Any suggestions guys? Please see my original post. Thanks!