If you are looking for simply a back up to your PLOG rather than a more complex system then even the simplest GPS will work well.
A Garmin Geko or eTrex can easily be loaded with waypoint and route dtat from a PC and can be used to keep you on track. Whilst it will not show you airspace and lots of pretty pictures, it only costs £100.
Now if you want to have GPS as an (unofficial) primary navigation aid then that is totally another question and another price.