How can I be sure the asking price for a business is worth it?
Businesses aren't worth what the owner asks for them, they are worth what the buyer is willing to pay for them. If those are the same amount, you have a deal, otherwise, make an offer.
I have a related post on my blog at blog.timberry.com from January of this year and there is a related post on Anita Campbell's Small Business Trends blog from yesterday. You can also search the web for "business valuation."