If you want something which will tell you when and where to turn (auto-routing), then your options are somewhat limited. Here are your choices.

1) Laptop. Buy any cheap GPS and hook it up to a laptop. DeLorme makes good software to run on your laptop. This option is only a choice if you have a laptop and carry it. Your cheap GPS could then go on a walk with you after unplugging one cable. This is your cheapest option if you already have a laptop

2) PDA. There are several PDA software and add-on packages. If you already own a PDA, you can buy software for it (try DeLorme), and either buy a GPS to fit it, or just connect a GPS up to it with a cable.

3) PDA. Garmin makes a Palm PDA with a GPS receiver built-in. Not rugged. Would be OK for an urban treck, but not for a backpacking trip.

4) Automotive GPS. These are made for use in a car. Examples include the Garmin StreetPilot III, Garmin StreetPilot 2610, Magellan RoadMate series, and the new NavMan machine. Expect to pay at least $700 or so for these. They also are NOT made for outdoor use. Some do not even have the option to use batteries, but MUST be tethered to a cigarette lighter. These are perhaps the best option for serious road warriors.

5) Garmin GPS V. This offers turn-by-turn routing, but can also be hand-held. This is a good all-around choice for car/outdoor use. The primary disadvantage is that this unit only has 19MB of memory for detailed maps. This amount of memory could not even hold most states. If you travel over a large area, carry a laptop.

6) Magellan Meridian. Magellan has recently added DirectRoute software to their lineup with will allow this unit to do turn-by-turn routing. This software has just started shipping, so I cannot comment on how well it works. The Meridian series uses standard SD card memory, so you can add 256Mb of memory for under $100. I have seen 128MB cards on sale for $40.

If you carry a laptop or big PDA, then using those is the car might be the best bet with a cheap GPS.

If you do not carry a PDA or laptop, but want the best turn-by-turn routing, and have some money to spend, then getting a dedicated car unit (> $800) and a dedicated outdoor unit (< $200) might be a good way to go.

If you can only afford one unit, then getting a GPS V or a Meridian would be a good compromise.