I have a personal Rails site that does something like this. There's an Expense model and a Expenditure model. Also a PayEvent model that's kinda the opposite of Expense, could probably roll them into one model. The Expenses are averaged out by month. An expense can be 'tracked', that means when I'm adding an individual expenditure, I can list it as that expense. My dashboard tells me at a glance how much I've spent on those expenses. If it's not tracked, it is factored in automatically.

It is multi-user, though I'm the only user at the moment. If anyone wants to give it a whirl, shoot me an email which you can find in my profile.

My philosophy is to avoid the banking APIs by putting in expenditures manually. I would love to integrate with OFX, but I don't have the time to maintain it.