Pam --
To add to Rod's excellent description on Expense Cost resources, you can
assign a Budget Cost resource to the Project Summary Task and specify an
overall budget for the project by doing the following:
1. Create a Budget Cost resource in the Resource Sheet view (you did this
already, right?)
2. Click View - Task Usage.
3. Click the Assign Resources button on the Standard toolbar.
4. Select the Project Summary Task (Row 0 or Task 0).
5. In the Assign Resources dialog, select the Budget Cost resource and then
click the Assign button.
6. Close the Assign Resources dialog.
You should now see the assignment for the Budget Cost resource immediately
below the Project Summary Task (the assignment is indented and italicized).
7. Right-click on the column to the right of the Task Name column and
select "Insert Column" from the shortcut menu.
8. Click the Field Name pick list, select the Budget Cost field, and then
click the OK button.
9. In the Budget Cost column for the Budget Cost resource, enter the
overall budget for the project.
That is how you assign an overall budget for the project. Now, see why Rod
Gill said it is ridiculously hard to use Budget Cost resources?

Hope
this helps.