If a product or service solves a small problem, what would make users transition from what they're currently using to solve that problem to this new product? There has to be some sort of incentive because inherently, we're all lazy and don't want to go through the entire process of switching existing systems.
Honestly, a simple group messaging app (or even email) + using Billmonk solves most/all of these problems. That's what my three roommates and I are using and there haven't been any complaints. As well, the more roommates, the bigger the need has to be in order to convince everyone to use this new system. The smaller the number of roommates, the even less necessary it is to have a new platform to manage all these affairs.
I think this is a cool weekend project, but would I call this a startup? I don't think so (at least, not by the definition Eric Ries uses in The Lean Startup).