This payment model would ensure shared server costs are payed for, without outright capitalism. It will find the total server cost for n time, and split that cost between all users that choose to pay. There could be an additional small fee to each user to fund development. This model will allow users with little or no budget to freeload, while generous users can donate to the pool.
This is especially useful for apps with known fees, like App Store fees.
Users can either be shown a bill based on the fees split between all users at the end of the billing period, or it could be upfront and refund unnecessary funds.
This system is comparable to Gumroad, where users can choose to donate.