We're using Fortran here. It handles 4 lines and 12 or so phones and does quite well. It also deals with voicemail and an automated attendent for after-hours quite well.
They're based in Newington, VA. I can give you the number if necessary. I'm not sure how widespread their coverage is, though.
As for cheap, I can't speak to that directly either. They got us on a State Contract, so I'm sure they just backed up an empty truck and the State filled it with cash. That's pretty much how the State handles everything (and the reason we're facing a 1.6 billion dollar deficit next fiscal year)
You can buy a used PBX most anywhere with phones for $50-100 at any used business equipment place. If there's a geek in your office send him/her to go buy a cheap used nortel or att/lucent pbx.
Telecoms are telecoms, mostly, so just get your best price. Chances are they'll be Chapter 11 soon enough anyway.
Buying or leasing new pbx gear for a NPO is silly if you can avoid it.