Yes, this would be great. For me, as an engineering consultant, I have a Purchase Order (i.e. A Budget) for each Project.
These purchase orders have a limit in $ based upon the proposal given to the client.
I have set up each Task to be a Project (I.e. a PO)
I export hours based on each Task/PO to SubTotal to create invoices.
Each invoice is submitted seperately according to the PO/Task.
I can't invoice beyond the PO $, so I must be mindful of how much I have invoiced to date (monthly invoicing), and how much I have left. (I manage this in Excel)
It would be awesome to have the ability to enter a Budget $ against a Task (&/or Tag, if that is easier), then the functionality to filter/track the billable $ against this budget.
This could be within Workhours &/or Subtotal, or even a separate App.
I have been using Subtotal and Working hours for about 4 months now.
It is so much better than the various spreadsheets I had. And way cheaper than other timekeeping/invoicing software that has many features I don't want nor need.
I have recommended ST and WH to other engineering consultants in my field.