You have a choice. If you get a "no frills" unit, all you get are coordinates. This is OK if you carry a map around too, but slightly cumbersome. They also offer basic mapping units for under $100 with simple base maps. For that money, you do not get all local roads, but you do get interstate highways and many (or all) state highways. You might also want to take a look at the lower-end eTrex (Garmin) and Xplorists (Magellan). Lowrance also makes some nice ones.

As a Magellan owner, I can say that if you want a full-featured unit, the Garmin mapping software is much better. But if you want something to throw in a BOB and forget about it, you can go for any manufacturer.

Just make sure that whatever unit you get takes AA batteries. Rechargable lithiums are not a good idea in a BOB.