I would say, until this project came along, $125-175 somewhere. Now that 600 odd people will be on the look out at the same time one might see a big hike in prices. My main motivation for not yet joining the bandwagon. I tried to get one just this weekend, and not a good one at that. Sticky slow speeds et al, and it went for $175.
Other 90´s can be applied it would seem. Though not the hefty ones with really big rear elements (like the Super Angulon f5.6 for instance). The helicoid infinity setting can be adjusted to adapt for lenses with different flange back registers, (all 90´s differ, as much as 10mm, from under 90mm to over 99mm). Those with longer flange back distances will as a result however loose a bit at the other end, not attaining as close up focusing as those closer to the nominal 90mm.
Ben states that the the following lenses could also be adapted, having in mind that some will loose more up close focusing than others:
Schneider Super Angulon 90mm ƒ/8
Nikkor SW 90mm ƒ/8
Fujinon SW 90mm ƒ/8 (my trusty lens already put to use on my 6x17 and 4x5 P+S - mounted in a helicoid)
Caltar 90mm ƒ/8
Rodenstock Grandagon 90mm ƒ/6.8
Caltar 90mm ƒ/6.8
Also note that most of these are bigger and heavier than the diminutive Angulon f6.8, and would have an effect on the balance of the camera.