You might find these links useful
click here (video)
I use a cheap F20 Navman,which at the time cost me less than £100 on special offer I think, and it does all that I ask it (ie gets me from A to B and back).So in reality all all depends on your requirements.
Tom Tom is a favourite with some people, but I know of an organisation that purchased over 200 of a particular model. Quite a number had to be sent back for repairs, but this in the main was due to user abuse.