[hhw]

Just determine how much the cost per user is, and make sure your business model works for that. Beyond that, I'd say not to worry too much about your total hosting costs as the reality is, it's impossible to predict how much resources you'll need until you're actually using them. Most of the time, people grossly overestimate how much hosting they really need on new projects.

Instead, I'd recommend focusing on being flexible enough so that you can start small, and upgrade your hosting as needed. Document your entire setup so that you can quickly re-create your environment when migrating to a new server. If your business model works when small with something like AWS, it's only going to work better as you scale up, as your costs will go down considerably as you get larger. You'll see a dramatic drop in costs once you outgrow cloud/VPS solutions and get big enough for your own, entire dedicated servers if you don't limit yourself to the really big players.