I think that the cheaper non-motorcycle GPS's on the market work just fine for a motorcycle. The only flaw with them is that they are not waterproof but it is easy enough (many of us do this...) keep a sandwich baggie or two in the cubbyhole box to slide over it during inclement weather.

My Magellan that I bought for 80 dollars or so from Costco is still going strong; although the power port for it is loose inside and I frequently have to reset it these days. May be looking at another "cheapie" GPS to replace it soon.