The lowest prices would be found on the "Asian imports", for example
http://www.harborfreight.com/cpi/cta...emnumber=43520
Mine is similar, cost me $20, and has worked for at least 5 years. On the other hand I have seen others non-functional, directly out of the box. Griot's Garage's version is $30
http://www.griotsgarage.com/catalog.jsp?SKUupsell=10215
But they would probably do more customer support than Harbor Freight. Some people prefer a combo device, like
http://www.harborfreight.com/cpi/cta...emnumber=39954
DAS