User can see its own "budget total"

We need to indicate how much a particular user owes within a budget. It's simply sum of transactions associated with that user within a particular budget divided properly:

2013-07-26, transaction 1, -$20.00, user1, user2
2013-07-27, transaction 2, -$5.00, user2

user 1: -$10.00
user 2: -$15.00

Note: We need to use rationals to calculate divisions properly. It's very important to do the calculations with infinite precision.