I think it can be a combination of subscriptions, per release donations as well as bounties. 

Subscriptions can provide private app support and other benefits. Furthermore, a percent of the subscription revenue can go to contributors of each release. For those that are more involved with development, each release recieves donations and tips, that are also distributed to contributors. And for the most involved in production, there can be a market of bounties.

Each I think provides an option to contribute value at different levels of involvement.