Diaspora is going to need more pods
Based on @Di Cleverly and @David Thiery 's experience and estimates, Pluspora is not going to be sufficient or sustainable for the anticipated G+ exodus.
They see 20-40 pods of 1,000 - 2,000 users each being likely needed. Not necessarily all at once, but this is something that should be getting considered.
Operating costs are on the order of $25/mo using cloud services, or about $0.025/user-month, give or take. A quarter per year.
This works out to roughly 50,000 users migrating over. My previous estimates of user activity, backed by independent analysis by Stone Temple Consulting suggested 4-6 million publicly-participating G+ users as of January - March, 2015. The active core of that may have been as low as 50-100k, which seems generally sensible based on other indications.
The numbers are much squishier than I'd like but at least establish a starting point.
There are other alternatives, such as protocols / servers which have lower per-user overhead, but we have no data on this presently.