If you are going to advertise free wifi, at least put the disclaimer in if its only availible near registration. I never expect free wifi to be fast or able to download tons of stuff. But if you advertise it to be availble at the campsite it should be a strong enough signal to connect.
I do know how much it costs to get good wifi coverage, but the price is dropping if the park owner does the research. Their abilities are improving as well to allow you to limit each client to control your costs and availible bandwidth. Again something you should expect on a free connection.
As for cost to provide the bandwidth for internet, that is going to range drastically depending on the location. In some areas you can do it for far less then 1200/m. But even at 1200/m in many areas you can price out some very competitive speeds for internet connectivity.
The problem is will anyone be willing to pay higher prices due to good solid wifi or pay a per day or per stay price to use it?