Assuming that each site as a DSL line or cable modem…
Get each site a LinkSys RV042. These are nice little routers which would replace the existing routers at each site.
These routers are nice because they support site to site VPN connections.
Each office is its own TCP subnet, and the routers are configures to all VPN to each other so that any traffic that needs to go from one site to another can. For example…
Site 1: 10.0.1.0-255
Site 2: 10.0.2.0-255
Site 3: 10.0.3.0-255
Site 4: 10.0.4.0-255
Site 5: 10.0.5.0-255
These routers cost a couple of hundred dollars each.
The servers are going to be where a lot of cost comes in. I assume you’ll want to use Active Directory for authentication so that any employee from any shop can log into a computer at any shop. You’ll want two computers to serve as domain controllers. You can do it with one, but if that server fails you’ll have a lot of problems. Then you’ll need something to run your mail server. Assuming that you want to use Microsoft Exchange that’s where a lot of cost comes into play.
There are less expensive solutions out there, but they can be more complex to setup and maintain where Exchange is pretty easy for a small company.
Without looking at server and licensing prices I would say that $70K is VERY high. Honestly somewhere in the $20k range is probably more reasonable. If I were to price out the job as a consultant I could of course give a more accurate number.