Out of curiosity, why do you need that? Why don't you just record the net price in the PO line instead of recording the gross and a discount. I am asking because we discussed about that in R&D and decided not to implement it because:

- if you change the net price to be the gross * (1-discount), you might have rounding errors and not have the exact same amount than your vendor. So, it's less risky to record the net price.

- with Odoo Studio, I recommend you to just add a gross price and ask your user to record the gross and the discounted one (net). That will be much easier than changing the behaviour of the current price field.