Hi Minhoru Cotache,
Use a calculated Attribute to keep track of the Actual Budget, and have an Attribute for the Initial Budget in a Budget Entity. All Costs booked could be associated with the Budget. If you enter new costs, check using an Event if Actual Budget has room to book additional costs. If you are entering costs as a list of Cost objects, you do this after each Cost object is added to the list for that entire list. Use the Aggregate list Action to calculate the SUM of the Amount in that list. Feel free to ask more details if it is not entirely clear.
Go Make IT