You will be surprised how useful a garmin sat nav with safety camera alerts (speed trap cameras both fixed and mobile) and real time traffic is, I use mine even when I know where I am going because it will alert me to my speed and dangerous stretches of road, and if I have told it where I want to go it will reroute and advise me if delays in traffic are going to slow me down.
Mine is six years old now and I regularly update the maps.
Personally I believe the costs are worth what they potentially save me in unnecessary contributions to the governments retirement fund.
My mother and my ex wife both had a tom tom before I bought the garmin, personally I would not touch tom tom with a barge pole.
With regard to the apps for mobile phone navigation I believe that while they are very good they are not as good as a dedicated sat nav, also they do not use satellites but the mobile phone tower signal to determine their position, direction of travel and speed etc so will not be as good where signal coverage is poor.
On another line of thought for minimal out lay you could turn your laptop into a sat nav, only problem is where do you mount it (size and so on), I bought one of
these and installed this
program on my lap top that I take with me when I travel just in case I need to find something online, I am surprised just how good it is but it is still not as intuitive as the garmin but will make a good backup.
If you have a tablet there are navigation aps/programs for them as well but most tablets do not have a standard usb port, those that do have a micro usb port and a few have built in gps so do not need a dedicated satellite receiver.
If you want a cheap sat nav then flea bay might be an option, or cash converters or similar but with them you take your chances.
When I change my satnav it will be because it no longer works or is otherwise useless.
Just a few ideas for you t1pper.