Steve is right, but this is how I go about monitoring the budget:
1. Enter your resource/cost info into Project (I even put in travel costs,
license fees, etc.)
2. Each month update the % complete field for each of your tasks. You may
have to play with these a little bit if you're looking at a bunch of
partially complete tasks and you have each task budgeted for a specific
amount, but you should be able to get very close...
3. Add the Remaining Costs field to the display.
I use this method all the time for PMing and it always gives me a reasonably
accurate picture of how much work is left to be done, how much that work will
cost and I then compare that to the dollars I have left to see if we're going
to make it w/in budget or we have to make adjustments somewhere.
Hope this helps,
Al