I received an email blast from Birch today announcing sub-$20 pricing on Hosted PBX seats.
There are certainly folks selling HPBX under $20 per seat, just haven't seen a national provider advertise it. And certainly I hear and see quotes with the pricing sub-$20 for deals during bidding as everyone wants to book any revenue possible. (The valuations seem to be about revenue and ARPU not profit margin.)
Have you read the 2015 US Hosted PBX Market report by RAD-INFO? Click here.