How big of an audience/revenue stream should a startup shoot for? I really have no idea. Just for fun, I ran some quick numbers:
A 10k yearly fee works out to 0.04% of MIT's library budget (26m), and 0.27% of my alma mater's budget (3.6m). If 10% of 4-year universities adopted the service (~230 colleges) that totals 2.3m yearly.
Obviously that is a really simplified example. I imagine larger schools would pay more than smaller schools, 10k is probably a silly number, ten percent adoption is probably overly-optimistic, etc. But I suppose it puts everything into rough context.
Another idea for marketing: You may want to consider trying to sell to departments by contacting faculty/grad assistants researching in the affected field. Then, have the faculty request the library purchase a license. We pretty much buy anything faculty requests within reason, but ignore salespeople.