The introduction of
Products in v1.2.0 with the ability to enter a purchase price is the first step to include a fully functional expenses module.
However I didn’t planned to include an expenses module into v1.x of InvoicePlane.
It’s planned for the next major version v2.x which should be released later this year.
A developer already started to work on an expenses module but never answered his original pull request. So maybe someone could take his work and finalize it.
The work can be found here: https://github.com/InvoicePlane/InvoicePlane/pull/371