The phones are the cheap part. You can actually use soft SIP client phones on a PC with headsets if you really have a money issue. There are a number that are free.
The big expense comes in the brain that controls this. Cisco calls this the call manager and mostly what it does it keep track of what IP is using what phone number and the if the phone is busy or not. Still this is not all of it. The other expensive thing lets you integrate into a real phone system. In general terms this is called a SIP gateway. But pretty much its purpose is to convert phone signalling between SIP and the protocols used by the telephone companies. Many times you buy the SIP gateway as a service from the telephone company. Some also offer the call...