This is why, people, this is why.

Eos; About how much would each active member have to donate per month if we wanted SP to stay up?

Really depends on how you define active, but generally $1 each would do it in most circumstances.
We had a whole thread on this subject once before.

That's rather bare bones though, and wouldn't account for trying to pay content providers like Locked, who make a portion of ad revenue as incentive for the service. Or trying to have events with actual rewards.