I maybe pro-outside shop biased because I used to work for one, but all in all I think they are a viable alternative when the store is too small to have a full-time person, or you need them as a stop-gap between employees. I think they do good at helping a dealerhip maintain control, but they aren't a substitute for good management in the store. If nothing else you'll at least know you'll get paid, so if the alternative is not getting paid for claims you are better off with them.
I am not sure exactly where the rates are at these days, but I would expect to pay anywhere from 3% - 5% of my warranty claims. (Disclaimer: the above statement is an estimate, moreover it's a guess, and it isn't intended to set the rates for such services or provide any type of guideline to what those services should charge.)
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** Rob, Editor WD&S **Help is only a message post away!
robc@dealersedge.com