Determining Cost of API Calls

I was going through the adwords API and came across their rate sheet - http://code.google.com/apis/adwords/docs/ratesheet.html .

They charge $0.25 开发者_StackOverflow中文版per 1000 API units and under the 'Operation Costs' sections list the cost (in API units) of different API calls. I am curious - based on what factors do they (and others) calculate the cost of an API call? Is there any simple formula to determine this?

Clarification: When I say 'cost of an API call', I don't mean the money but the API units. For example, how do you determine one API call costs 100 API units and another 1000?

Note: Those interested should also refer to my cross-post for the same question in PROGAMMERS; the answers there were really helpful too.

This really depends on how the company views its paid API and associated charges - as a product in itself, or as a way to recover costs of providing the API.

If it's a product in itself, it's just like pricing any other product (e.g. charge what the market will pay, etc.) There's no formula - it really depends on what you're selling through the API.

If it's just a way to do cost recovery (e.g. of running the API servers), you determine the average relative costs to you of each call. For example, if you know that a certain call would usually consume 10 times the CPU resources of another call, and if you know that CPU utilization is the bottleneck in your system (i.e. the thing that may force you to buy more servers), you price that call at 10 times the other call.