I'm single-running an Atlassian add-on, $4000 the first year, $40k the second year, $55k the 3rd. APIs are young and change often (=maintenance costs), market is not saturated yet, but the competition is competent. It's a real benefit (=3 month savings) to your startup if you don't have to build the sales engine: I've sold to very, very famous names, that I could never have had if it wasn't through Atlassian's appstore. If you're thinking about coming in:

- Build a real, big product, after doing some customer interviews,

- (but get revenue from month #1, obviously, as any competent startup),

- If you go for a Cloud add-on, maybe think of doing software that can be used independently,

- and there is the Codegeist competition currently running on, until something like October! It's designed to get newcomers. Think about it, winning is a great way to launch! (And it's every year)