I'm building this mentoring web app and as much as possible I want it to be free. Initially I'll be using my own money to set it up but I also don't want to keep paying for the infrastructure costs (servers, etc.).
About the app, basically there are two actors: mentor and mentee. Both can can decide wether they want to be mentor/mentored for free or with a price. This will help a lot of students and fresh grads who are willing to work free just to gain experience. And likewise for the mentor to build relationship with future colleagues.
So I'm looking for a self sustaining business model that would keep the site up without asking much from the users.
First I thought of donation but there's no guarantee in it and it would be sad to shutdown the site just because I can no longer pay the bills.
Building your application, serving your users, and showing your traction should be your first objective, leading you afterwards to find your business model. We could take for example Amazon or Twitter, which struggled to find a sustainable business model in their early days.
In a world where more and more startups launch and usages are evolving it can often be painful and time consuming to find a business model.
You should launch your product, see how users are using it and find a business model that suits users' needs and cover your costs.
How much is the hosting going to cost anyway? I wouldn't expect more than $100/month. You probably could make it work with $10/month if you had to. Consider that a fair option.
Donations will not work. It takes about 1000 active users to donate $5 once. And then everyone is upset at you and feels entitled.