Dear Sir/Madam,
I would be surprised if I am the first to point this out, but nevertheless I cannot find it in the community.
When creating sales items, could you also add a field where the purchase price can be noted? Personally, I think this would be useful when you want to give a discount, for example, but obviously need to keep an eye on your margin.
Or when you have simply forgotten how much you purchased a particular item for and someone is interested in it/you actually need it.