As already said on the Ethics page, there are different kinds of ISPs. If you want to make your own ISP, you can consider the following business concepts.
This is the classical access ISP model, which the FFDN in France has. You have some possibility to provide access to people, and you have them pay for it. This could be done via Wifi or fibre (most probably you set up the fibres yourself) or DSL (which is most probably DSL reselling).
This is the easiest model when it comes to calculation: The access you provide is directly paid. Though you will have some base fee, you can directly scale the number of connected users to the income of the ISP.
This is the model of guifi.net in Catalonia, Spain. They have a large network of (partially) interconnected nodes. These nodes are connect via wifi or even fibre and build up a large community network. Inside this network, there are providers which provide (against money) several services, also containing Internet access. But if you want to contact somebody else who is on the network, you can directly contact him without having to pay for anything.
The model that most of the old Individual Networks (from Germany) have running nowadays. Since DSL from the commercial ISPs became very cheap and it is difficult to still sell DSL connections (both because of the technics and the contracts), most ISPs have ceased or are ceasing from the access market. But there was already a lot of members using their mail address and webspace from that provider, so they moved to just being a hosting provider. Starting a provider of this kind is much easier, since you need really low resources: You only need a server somewhere to start offering services. And financially, this is in the beginning the only base fee you have. Every member of your ISP will cover that, until eventually, you might have enough money to run a second server, etc.
If you are large enough, you can consider hosting VPNs. The current net-neutrality issues often target only the large service providers like Google or Facebook, not the small ones (though we will see in the future how this will be implemented in detail). Renting a DSL connection from a commercial provider and sending all the traffic to your ISP before going out into the wild. This way, you will have unfiltered and neutral Internet access, encrypted up to the point where it leaves your network.
You'll need lots of traffic to sell VPNs for good price, but then, you might be cheaper for you members to get a DSL connectrion from any commercial ISP and just put a VPN on top of it.
If you are providing VPNs or are considering providing VPNs, please have a look at the Working together page – we might be able to share VPNs to offer a broader range of VPN endpoints to our members.