start small and keep overheads low, whilst there might be a temptation to go large early it places unnecessary financial strain on teams in their start-up phase. You can always grow bigger from a small base, but its hard to cut costs once you have already committed to contracts.
They've publicly stated that in terms of attendance their sweet spot is somewhere between 3-7,000. It's part of the reason they went with Sacramento. PNC (Pacific Nations Cup) games leading up until this year have consistently either sold out or fallen just shy (this year they were played on a Friday during the workday, still around 10,000 turned out). Evidently, stats suggest that the vast majority of those who attend are Sacramento locals. It has something like 8,000 active players most of whom are in youth leagues. It's a strong a rapidly growing Rugby area.
People on their FB page have been why they didn't go with Avaya Stadium (18,000) in San Jose as opposed to Boxer (3,500) but someone seems to give away a possible reason as Avaya has publicly stated they want to host one of the Super 7s Rugby teams if that ever gets off the ground.
All the financial strain is on ProRugby's owner. He's funding everything. Which is interesting as as the player wages for the season will range somewhere between $3.8-5.4m depending on how many are on $40k contract etc. Take into account travel, accommodation, facility hire, staffing, events etc. and if he's extraordinarily frugal he'd be lucky if it doesn't cost him at least $10-12m a season in it's current form.
I think what may in the end make or break the financial side of this is the deal they strike with their broadcaster (most likely the Rugby Channel that's in the process of being launched by RIM) and the share they receive from subscriptions generated by those wanting to see it.