I'm currently working for a client that is rolling out VoIP using Cisco gear, phones, etc.
While VoIP saves money in the long run, it does require what can be a substantial up front cost.
My recommendation would be to hire a reputable consulting firm to look at your business needs & do an analysis for you. At least then you'd know for sure what kinds of costs you're looking at.
I can't solicit here, but if you'd send me an email, I may be able to help you.
------------
Bill
Consultant / Network Engineer
CNE, CCNA