This is actually not possible to have a proper system to allow this.
This is two majors problems:
- The item price text can't be changed, whatever old and VGUI menu (it's either harcoded in config file or in client binary)
- Once you open VGUI menu, to know if user has enough money to select an item, client relies on the current HUD money value, this means, since we can't directly changed price, before opening the menu, money needs to be somehow faked and restored once closed. It's awful.
Overall it can't work properly, and at the end current forward doesn't make sense.
If an author wanted to have its own prices, the only only way would to force players to use old menu, then overwriting the whole buy menu, so you would be able to display what you want exactly.