The Nuvi line used to include waterproof models that are far cheaper than the true motorcycle units. I have a Nuvi 550, but it is discontinued, and I'm not sure what the replacement is.
A few things you should look for that not all car GPS units will have:
- Water resistant
- Ability to create custom maps with Basecamp (may not be important to you, but I think it is for most riders)
- RAM mount to give you plenty of mounting options
If I was going to give up a feature, it would be the first one, water resistance. Tuck it back under your windscreen, or put a plastic bag around it.
The Nuvi 500 and 550 used to be a good way to do this without going to a full motorcycle GPS, so you may want to do some searching around for replacement models for them.