Does anyone have any suggestions for ensuring a reliable marketing agreement evaluation?

Could you be a bit more specific what you are looking for? Is this in the context of a RESPA related Marketing Service Agreement? A prescreen offer? Something else?

RESPA Section 8. Looking for suggestions on how to get an acceptable value or a third party source for a value.

It really depends on what services you are trying to get a value for. You are looking to find the value that someone outside the market would pay for the same service so the easiest way to find your value is to look for a similar service outside. So if you are looking for web marketing value, look at PPM pricing for similar websites or from google. If you can find a marketing company that you can get a quote from on similar services that could give you a good basis. I know some law firms will also assist in the valuation support of these things but you would need to make sure you are getting a good RESPA attorney not just a general bank attorney.