i use a mio digiwalker. i paid around 30 sovs for it from ebay which is about the going rate, i load up where i want to go, put it in my pocket and feed an ear piece through my jacket and listen to the instructions. cheap, effective and waterproof when in a plastic bag. i can't figure why people pay hundreds and festoon bikes with ugly mounting fixings which necessitate taking eyes off the road when this solution is available.i also use it in my work van and mrs uses it when cycling places too.down sides? it can sometimes take a few minutes to find where it is. it isn't a full postcode search, part postcode and then road name are required. the supplied ear piece was very uncomfortable so i bought a decent in-ear headphone set.
I've got a crappy £20 unit which isn't technically waterproof, but has never had a problem in the rain. It's custom mounted on the (dark) screen and works through my autocom unit. Perfik and not too worried if it takes a swan dive off the bike at that price (I have my phone as backup if all else fails). No way I'm paying £££ for a unit which has 'motorbike' in the title, but is otherwise the same as the cheap units
Mine fell off .the pin in the top clip came out,clip fell off and so did my Zumo,be warned
So last i saw in my mirror was my Sat nav bouncing along at about 130mph,,didnt bother to stop
or on the cheap you can use any sat nav just get a waterproof bike sat nav holder which comes with handle bar fittings put a 12v sockect on your bike again very cheap and there you have it one cheap bike sat nav