This is the cheapest per serial port I have seen to date - Sena PS810
It's very easy to communicate with via UDP or TCP. I have a project with five of these that have been working flawlessly for nine months.http://www.sena.com/products/device_servers/external/hd_ps_x10/
I used Extron and Moxa serial port servers on other jobs. Sena has the best value.
It's been said before:
A slaved Axcent 3 bought on eBay is EXACTLY what you are looking for, solid as a rock, extremely configurable, no significant code changes required, and doesn't even use a serial port on the master.
An "AMX NXI w/ME260" from ebay would also work. A bit faster than Slaving an Axcent3 via Axlink (28k baud).
Yeah... and How much??
You are right... but how much it may cost a used axcent master??
as cheap as 300USD??