This guide doesn't pretend to be unbiased, nor is it exhaustive.
The basic argument here is that :
1/. If you pay peanuts.....you are highly likely to have purchased monkeys.
2/. When you are looking into buying your first SatNav, it is reasonable to conclude that, as like myself, you will not will be wholly aufait with the most important characteristics which will make your purchase something which will get you to your next destination quicker, or will help you avoid driving into the river, or, probably most importantly, give you great directions just before you reach the left hand turn, rather than 10 seconds after you passed it on the one way system, or even worse, after you passed it on the M25 or similar.
I have now, (think bolting horses and stable doors), discovered the downside of buying a cheapo SatNav. Don't go there. Fortunately, I have now been gifted an excellent TomTom Go 720 for a birthday. I have to say: you only appreciate what a great machine can do only after committing Her Majesties folding.
If you are tempted to save some cash......please read as many reviews as you can until you are cured of the temtation. You will thank yourself for the time you have spent.
Buy cheap, pay twice.!!!
Good luck and happy travels.