My dad has gone through a few. We found that all of them vary in the end. Some of the $40 ones didn't work well, and then some of the cheaper ones didn't either. Have you tried any yourself? We finally settled on one of the cheaper ones that works brilliant. It's cheap though, which in the end doesn't really matter since it works. Better buying 2 of those than 5 $40 ones, heh.
I'm selling the official Apple FM Tuner in this thread: http://forums.macrumors.com/showthread.php?t=1500802 PM me if you want more info.