Amount in project-based lines of Invoice not being calculated

I've created an invoice from the Project Contract. In the project-based lines section of the invoice, the correct line item is listed, however the amount to be invoiced is zero (when it should have some $$ because actual work has been approved) (refer Capture 1).

When I go into the invoice line detail, the actual work that's been approved is listed but there's no price or amount totals (refer Capture 2).

I've checked the project contract, and the correct project price list is identified and activated. Has anybody come across a similar problem? Or any ideas how to solve this?