Just like anything else in IT… the answer is “it depends”. You can have a basic system, a system with some options and a “decked out” system. The minimum components you will need are:
Phones – $150 and up
Cabling – CAT5e or above – how much per drop?
Layer3 Switches (Power Over Ethernet – POE – preferred) – have to also provide Quality & Class of Service (QOS, COS) – $3000
Gateway between data network and PSTN – varies, low end maybe $4500
Callmanager – can be integrated into gateway; this is the “smarts” of the VOIP system – varies from free open source (Asterisk) to high end systems like Cisco which could run $15,000
Software licenses if not using open source options like Asterisk – say $100/phone
PSTN connection(s) – copper trunks, PRI, ISDN, etc – depends on your local telco
WAN costs for site to site VOIP – may have to add capacity and/or QOS/COS to this link – depends on your provider
So, you can assign a quantity and price to each of these elements that will get you your estimated costs for the boss.
Call charges .
Mobile or Cell ? ( GSM gateways) or the like
Line rental saving if using VOIP lines
Versa capital expense of putting PBX in
+ Extra functionality
Will lead to a bus case of do it or forget it
If you need further info drop me a line .