The first one that comes to mind is the BC246T it may be the cheapest since it has been discontinued. Others may be better but you could get one for a good price through someone may get lucky and someone may still have some new ones in stock at good prices as well.
Okay.. how comparable in quality is RS's Signal Stalker? I'll post the question in the RS forum, but I'm curious if anyone knows... I've always been a Faithful Uniden user, and I'd feel like I'm cheating
The BC346T is still being manufactured by Uniden. I have seen that on line for slightly over $200 and have seen some online sites selling the BC246T for the same amount as the BC346T. I Have the BC246T and bought it when it came out but if I had to choose between the two today I would go with the 346T because it is newer. Like the BC246T the BC346T also has the close call feature.
I have never dealt with any of the online sites that sell these as their is a dealer that is local and have gone to them for complete systems but I am sure someone here has and they can recommend a good online company to you.
An even cheaper model (assuming that you don't need trunking and digital capabilities) would be the BC95XLT. I had the BC92XLT which now I think is discontinued. I believe it sells for just slightly over $100.
Don't forget the Uniden BR330T. Little older radio that many people think of as just another Nascar but it has close call capability. Works good and can find at reasonable price on this website or ebay.