I'm thinking the marginal costs to providing this service, as well as the marginal costs for providing FogBugz (and many, many other SAAS apps asides), are so low as to be almost incalculable. Every customer is profitable almost by virtue of being there.

Also, in a company which is growing rapidly, the average price per month paid at any plan level is going to quickly converge on your currently published price per month. That is another factor which suggests using Joel's solution, if you're growing.