Yeah, Hellfrauds are dumping a load of old stock cheap.
Do you need mapping updates? Its worth keeping maps relatively up to date (bloody useless paperweight if the maps are out of date), but consider if its just worth buying a new entire unit every couple of years - often new unit (those crappy portable types) not much more expensive than map updates.
I still reckon Tomtom are the best of the crappy, low cost portable units. Based on hardware reliability, software reliability, route planning (none are perfect) etc. And I've used most of the brands at some stage. Navman aren't bad to use, but unit reliability seems suspect to me.
If you have a smartphone, that might open opportunities - Mrs TheBoy got CoPilot for her phone for under £30 for UK and Eire. Certainly flawed in one respect (can't plan a route in advance if you dont have a sat fix!), but works reasonably well on the road - certainly equal to any portable device.
None are a patch on a 'proper' built in system. Even older ones like CARiN outperform portable units, even if they do lack features (like speed cameras, postcode entry). 2nd hand CARiN for Omega would be around £60-£70, and not too bad to fit in saloons. Sadly, no more map updates beyond 2009 for CARiN, and the system is slow to plot a route with latest maps.
Facelifts use the rarer, more expensive Siemens system, built into the headunit and MID.
Another advantage of built in systems, less junk to put in your missus' handbag. And your car will be broken into anyway, as the theives look for telltale 'circle marks' on windscreen, and will want to look in glovebox.