The sweet spot for me would c.£100 a vehicle, plus another £125 for sound. But I would very easily be persuaded to go much much higher for any of my special favourites...
Any of my dream units would probably see me ordering two sets for a start. I'd want a pair of TfL Rail 315s. From an Electrostar release I'd kick off with 2 Hex 387s and probably a GWR 387. And if canny tooling made a D78 Stock / Vivarail D-Train release possible I would be in the market for 230001 in GWR guise and a full LU rake.
Which makes me wonder - if AS chose a longer prototype how would that be split? For example would a Class 700 come in a megasized 12 car box, or as a base set of say, the two driving vehicles with a trailer or two and additional add on coach packs available separately?