I agree with "who would need an external antenna for a GPS". Regardless whether the price is right, you need to ask yourself if you'll actually achieve any better reception than you already do with just the GPS unit itself.
Satellite signals are "Line of Sight". Which means it does not take much to interfere with them. Kind of like a satellite television signal where a simply tree blowing its leaves in front of the antenna will screw with it. If your concern is the signal you'll get on a bike, spend your money on some other farkle. If you lose your signal while riding down the road, chances are, an external antenna probably won't help in that situation anyway.