The "cheap" ones you've seen are indeed transponder echo based but they don't give you azimuth (bearing) information which IMHO makes them useless because one cannot usefully act on the warning.
Those that give you bearing info start at about 15k, installed.
Unfortunately, so many people fly without a transponder, or with a transponder switched off (accidentally and often deliberately) that the warnings you do get are just the few.
Most UK GA is done at very low levels, say below 2500ft. Higher up, the proportion of "serious" pilots goes up and transponder use is much more common, but then there is far less traffic up there.
Just my view. I'd save the cash for the best possible moving map GPS - that will transform your flying confidence and ability to go places
(time to duck)