It all really depends on how much you want to spend, where you're going to use it, and what features you want.
You can often get a used 12 channel portable GPS either on Ebay or Craigslist for anywhere between 100 and 200 dollars. These are often older machines or simply less capable than others (meaning either slower, older maps, maps have to be uploaded for every trip, less powerful tracking of positioning satellites, etc.). This doesn't mean they're not good systems. If you get the right combination of features you will get a lot out of it for less money spent.
New portable navigation systems will cost anywhere from $200 to $1000. The advantage that portable GPS systems provide... you can move your GPS to any vehicle (useful if you travel a lot and rent cars wherever you go), the battery powered models can be taken offroad (say for instance if you like to hike, though there are GPS devices better suited to hiking than these), and they are much less expensive than having to install a unit in-dash or purchase a factory installed navigation system (Acura Navigation System is a $2400 option).
Features to look for:
-Maps that reside on the unit and do not require the user to upload maps from their computer for every trip. These maps are stored either on a hard drive or a flash memory card. These days even less expensive models have this feature.
-Strong mounting arm. Wherever you decide to mount your display unit you should get something with a sturdy mount that will stay attached and will be able to withstand constant summer heat or the continual jostling of regular driving. Suction cup mounts need to be dependable as you are going to have sensitive electronic hardware worth a couple hundred dollars mounted to the end of it.
-Touchscreen operation
-Latest map updates and USB connectivity for firmware and map updates
-Portability (size)
-Map View vs. Virtual View
-Real time traffic updates (requires service subscription... usually about $60 billed annually).
-Address book
-Intersections and Points of Interest
-Automatic rerouting
-Voice prompts
-Text-to-speech (onboard computer phonetically "speaks" street names so you don't have to look down at the display)
There are a ton of other features like MP3 connectivity, et al.
In-dash units are usually DVD-based and require that your dash will accommodate at least a single-DIN device. Companies like Pioneer make these systems integrated with a CD player and radio tuner as a replacement for your car stereo system. These do not require you to "hide" the unit as portables do since they are portable and easy to swipe. But it is fixed and will reside in whatever car you install it in and nowhere else. The displays are usually motorized and somewhat larger than in portable units. Costs of purchasing and installing one of these units are on average a bit more than the portable. But really if your preference is for a fixed system then this is certainly cheaper than the factory option.
Factory navigation systems provide the most aesthetically seamless installation for an automotive GPS. Since the system is designed around the car, you can be sure the design, fit and finish will "match" with the interior of your car. Some manufacturers also feature voice activated controls for navigation systems. However, like the in-dash option, you cannot move this one from vehicle to vehicle, and unless you actually purchase this with the vehicle, this option is prohibitively expensive.
The best way to know is ask the experts and see theirs and customer ratings. Check CNET.com and other consumer product sites for more information and reviews. Keep researching and find out as much as you can.
BTW, inexpensive units... I like the Magellan Roadmate 2200T or 2000.
Navigation systems generally cost the same wherever you buy them... try Fry's, Circuit City, Amazon.com, even Walmart. Find the model that you like and then shop around. Tiger Direct will have great deals on a refurb if you don't mind refurb.