"Roimisher notes another big issue involves smooth operators who'll markup list prices in excess of the rates published on data cards. But that's an easy one to check up on using NextMark, SRDS and other directory sources."
Wayne is right: it's very easy to verify prices. Especially, with NextMark's mailing lists search tool. It gives you free access to a comprehensive directory of information (including pricing and counts) on postal mailing lists, email lists, telephone lists, package insert programs, and more. No registration is required, either. Check it out!