There is a software option that is $600 less than the hardware solution. The software runs on CentOS 5 or 6 (I'm assuming RHEL would also work here). I was able to quickly stand up an un-licensed version (doesn't actually do anything) to test with in a VM on my laptop and I didn't see any resource problems.
I suspect one could easily run this on a 1GB Linode for $10/mo and have really good availability. The best part of using a hosted virtual machine for this type of system is that you can duplicate it (to have a hot backup elsewhere), geographically move it around (to counter physical disasters) and be afforded a great deal of availability if needed (of which you'll want since this is a radio system you're dealing with). Plus it's easy to increase the resources afforded to the VM if needed (as the system expands to allow more repeater connections).
I may be incorrect on my assumptions, and if anyone has tried it I'd really like to know, but I think this sounds like a good and cheaper way to deploy a c-bridge.