The Impreza 2.5i starts at $16 995 in the States. If we assume for a moment that ToyBaru takes the simplest route and replaces some sheet metal on the Impreza (a fairly simple matter, really) and drops a more powerful engine in it, it should be entirely possible to price it below $20 000. The Impreza (and Legacy, if they went that route instead) platforms are already setup for RWD - tuners have modified their Subarus for RWD, and from what I hear getting the car to drive only the back tires is relatively straightforward. Apparently, though, the car has no traction, since the suspension is setup for AWD and needs complete revision.
But really, if ToyBaru is using an existing platform and tweaking an existing engine, I don't see why they can't price a RWD Impreza coupe near the $20k mark...