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