I buy used Garmins off of eBay. You can usually get a decent one for $75. If it starts to rain, use a ziplock bag like someone else suggested and if it gets ruined you can still buy 5 or 6 more before even approaching the cost of one of those overpriced, motorcycle specific models. The one I am currently running has lasted over 3 seasons now and been through numerous road trips in various weather conditions.
2017 FLTRXS - Road Glide Special (Olive Gold): Cycle-Rama CR-460 cam, Fuel Moto Jackpot 2-1-2 head pipe, SE Street Cannons mufflers, SE Ventilator air cleaner, Dynojet Power Vision, Freedom Shields 10" Sport Touring, Rockford TM400x2ad amp w/ BT line leveler, Arc 602 speakers.