In Price Book > Prices for Stock Item you can only specify a price for the base stock unit, so the user cannot specify prices for particular UOMs.
Quite a lot of customers would like to provide specific prices for UOMs on a stock item and have this reflected when the UOM is selected on a Sales Order Line.
There is a workaround by using Special Prices against that UOM, however that relies on the discount being linked to the customer group and all customers linked to that group. It is also quite a long work around and relies on the setup being correct. It would be much easier to be able to set a selling price for each UOM within the standard selling price screen.