Selling my business...how to determine price?

I'm not sure if I have put this question in the right area. I own a food wholesale business and would like to retire within the next 2 years. I've chatted with a few business brokers about selling my business but when it comes to determining the asking price, I just don't trust what the numbers they provide. It's seems fishy that they can simply apply a few generic formulas to really come up with a determination of what my business is worth. Perhaps I'm biased because I've worked many years to grow my business to where it is now. I would like to have a third party value my business and when I research online I find it a bit overwhelming. I don't particularly want to spend enormous amounts of money for what should be a relatively straight forward value determination; however I would like to have a thorough and quality assessment made. My question is, does anyone have any experience with any of the following 5 providers and if so, would you be able to comment on their respective services?

http://www.strategicvaluationassociates.com/crosscheck-business-valuation-report.asp

http://www.shannonpratt.com/

http://www.fairmarketvaluations.com/

http://www.onlinebusinessappraisal.com/

http://www.acevaluations.com/

I've spoken to reps from each and so far the last one seems to be the most helpful. If anyone in the community has any tips or advice with respect to having a business valuation conducted, I would really appreciate it.

Thank you kindly,

Darren