Yesterday I found a 620-format Bessa from fleabay with a price tag of 250USD. According to the seller that is priced to sell. Made me wonder if stepping to 620 was a bad move and that good cameras aren't that cheap.
I suspect that good high spec 620 cameras are quite rare - and therefore might command a price tage above and beyond what a 120 version of something similar would get because of collector interest. Useability doesn't seem to bother collectors...
I've just had a play with 620. In my case someone gave me a cheapo budget camera, a Kodak 620 Junior, as a present. It has a primitive but interesting 'Twindar' lens. This apparently was designed to be as cheap as possible as it doesn't use any expensive optical glass but is basically a pair of simple meniscus lenses with a stop, although the front one has an extra focussing element.
I rewound a 120 film onto a home made 620 spool to try it out. My experience is that rewinding 620 is harder than you think. It sounds easy... but it is difficult to keep the film and backing in step - I ended up with a 'bubble' of film just before the tape, so had to remove and re-tape. Not very hard, but a bit more fiddly (in the dark) than I was led to believe. This meant my film was now slightly out of register with the numbers, so I chopped the end off one frame. I also seemed to get a lot of dust, a few stress mark and the odd finger print on the film. All stuff that could be sorted out, I suppose, after a few attempts - but it wasn't plain sailing.
Personally I wouldn't want to be saddled with the hassle of rewinding films all the time for more serious use. One off to try out the lens was worthwhile, but I doubt I'll do it again.
(BTW the lens seemed to perform adequately well - contrast and resolution are both a little soft but with excellent coverage to the edges of the 6 X 9, with little light fall off. The camera itself was awful because it has no proper viewfinder but relies on a tiny little 90 degree 'brilliant finder' only, so that one can go into the display cabinet!)
The bizarre thing for me with 620 is that the reduction in size is so small as to be negligable, it really doesn't make the cameras appreciably smaller - barely a few millimetres. I'll go with Dave's theory... a marketing ploy.