Slice it down enough, and the average user can donate without much issue
Slice it down even further, and it can simply be the cost of entry. In the same fashion that P2P sharing communities usually require that you make X gb of data available before you can join, or closed torrent communities require an even seed/leech ratio, your federated network can require that you allow upload and 10gb of hard drive space to use (or vary based on usage or something)
Distribute the cost to the point that for any given person the cost is negligible is the ideal.
Slice it down even further, and it can simply be the cost of entry. In the same fashion that P2P sharing communities usually require that you make X gb of data available before you can join, or closed torrent communities require an even seed/leech ratio, your federated network can require that you allow upload and 10gb of hard drive space to use (or vary based on usage or something)
Distribute the cost to the point that for any given person the cost is negligible is the ideal.