Wendell,
I researched this extensively as I really wanted to go with VoIP as the options on configuring it daily via the web were incredible. I am not sure what each hosted provider would charge, but we had to have Power over Ethernet (PoE) switch $1,000, there is a VoIP-type control that I can't recall the name that is around $2500 or more. That is the brains. You can't use POTS. You MUST use VoIP capable phones. Cisco, Toshiba etc. make these. They run around $150 to $350. The real cost is each phone seat. It is completely different than a PBX phone system.
With a "normal" phone system, you decide how many lines you need. Say each line cost $25 monthly, and you had four lines. That is, of course, $100 monthly. Now, you could add 100 phones with extensions to those four lines, and it would not cost you anymore. Each seat (VoIP phone) runs between $35 to $75 or so depending on what features you want (Outlook contacts availability and so on). So, say you went with level 2, which would be $50 per seat. 100 phones would run you $5,000 per month.
Now, of course, you won't have 100 phones, but we have six. We pay ~ $30 a month for two lines or $60 plus a fax line for another $20. So = $80. If we used VoIP phones, we would pay ~ $300 a month plus our $20 fax line, which is also a backup line if your Internet goes down or power is out.
The "brains" has to be a decent one as you want QoS (Quality of Service) if I have the acronym correct. This measures your bandwidth constantly. Again, as in a previous post, you need around 512Mb both ways. If someone downloads a large file, you could lose 300Mbs for five minutes, thus having low quality reception but still good output. Having large bandwidth gets around that problem, but most people don't think about the upload speed. The QoS makes sure that the VoIP phones take complete precedence over the Internet usage.
To be fair, I have not looked at or researched some of the VoIP recommendations on here. But, my recommendation would be at least strongly check out an actual hosted VoIP solution. While someone who works for that company has an agenda, they could at least explain the differences. For a business solution, where you completely rely on your phone system for communication, there is a reason for business-quality VoIP.
Hope this helps.
