I have heard very good things about Garmin's nuvi series GPS units. Although I don't know if you'll be able to find a new/used model under $100, the nuvi 200 can be had for ~$150 on Amazon or maybe less if you shop around.
I am just starting out geocaching - (actually Jeep-o-caching in my case) and I am looking at a crossover unit. Magellan seems to offer one, and somebody offered one for sale here but I missed it!! Anyone have input on that type of unit? It might be what you are looking for. Seemed to be sort of basic - but it has the added bonus of being a crossover. Take it off the dash of your car and go geocahe, then move it to the boat, etc.