How to price code reviews to encourage good behavior? [closed]

Question

I work for a company that has a hosted .net internet application with many clients. Those clients often want to write customizations for our application. We have APIs to hook into the app, but the customizations themselves are written in .net. This is a shared, secure hosting environment and we have to code review these customizations before we can deploy them in our datacenter to ensure that they don't degrade performance, crash our servers, or open any security vulnerabilities. We charge for these code reviews.

The current pricing model is simply a function of the number of lines of code. I think this is a bad idea for a variety of reasons, but primarily because, if we are interested in verifying that the code works as expected, we should be incentivizing good, readable code, not compaction.

I would like to propose a pricing model that incorporates some, or all of the following as inputs:

• Lines of code
• Cyclomatic complexity
• Avg function length
• # of functions

Are there any other metrics I should incorporate, or other ideas for how we can reasonably create pricing for code reviews that encourages safe and understandable code?