Hi Nick,
You're running into the origin of the Workfront-pertaining phrase "you can go down, but you can't go up".
Natively, the closest you can get to what you're trying to do would be to iterate all tasks from the project and show only those that have a Total Invoice Amount, but iterations can only display information, not aggregate (e.g. sum, in your case).
Given that, you could create a project level custom parameter (e.g. "Sum of All Total Invoice Amounts", with a currency datatype), then on a dashboard, show a project report at the top that exposes that parameter, a task report grouped by project that shows the total by project, and (as often as needed) manually type the latter into the former. With that data at the Project level, you can then continue on with the other cool things you have in mind for it.
With that process then working, once the novelty of the the manual data entry component wears off (or you make a wrong decision based on stale data; whichever comes first), I invite you to consider our UberCalc solution, which is is a Swiss army knife of functions you can use to enhance Workfront, such as generating job numbers, aggregating data, designing custom tabs with specialized workflows, and much more.
Regards,
Doug
Learn more at www.atappstore.com, email doug.denhoed@atappstore.com, or tag me here @Doug_Den_Hoed_AtAppStore.