A really cheao solution will be getting a couple of Ericsson Web 100 boxes. there is someone selling them for $220 for a set of two on e-bay.
The guy seems to have a large number of them.
I bought a couple of them. They do not need a gatekeeper to operate and they have four FXS ports .
You could connect the boxes to the PBX ‘s using analog two wire trunk ports.
The audio is super and the setup is done via browser in about 15 minutes per box.
If you want something suported, you could get any gateway, two or four ports,for around $200 to $400 dollars each.
Quintum will be a nice solution, the two lines (A200) runs about $400 each. The four lines sells for around $800 each.
Or if you want real good quality, something profesional, you will need something with a T1/E1 interface to the PBX. That will take you into the several thousand dollars to begin with.
You could get it cheap, realiable or fast. Pick any two .