In this case, it's hard to give advice because you want the tablet you get to operate from a car. That will mean getting a wireless Internet access plan. I would recommend looking at something like the iPad or Nexus 7 with wireless Internet plans and without (ie Wi-Fi only) and see what the difference is. A car GPS may be cheaper.
You're going to have trouble finding a decent tablet under $200. The iPad mini and Nexus 7 are over that amount. When you go below $200, you start getting into specs you don't want. Also be careful because many of the cheap Android tablets lower their cost by not paying Google for its APIs, which include the maps, so cheap tablets may not do what you need. Many Android tablets deserve their bad reviews. The Nexus 7 tablet is an exception, because it has better specs.
I'd look at the $250-300 price range for a decent tablet, and add a wireless Internet plan, and see if you can pay that much. You're going to be pushing $400, plus I think the wireless Internet is a yearly plan. (I don't know, I just have Wi-Fi.) If $350-400 is too steep, it may be time to just get a car GPS.
(Also make sure the wireless Internet will work inside your metal-frame car if you want the tablet to work as a GPS.)