The Item record is the same for all varieties of an Item, and the cost price is a part of the item record.
Regarding the sales price, it is not possible to set different prices per variety. I suggest you to report a development wish
More explanation @Paul:
When the issue occurs, should see if restarting presence or lookup helps. If so - that is the issue and should be sorted. First need to ensure that that is the issue...
Hello!
You can mark it as closed as option. If document is connected with other documents you can not delete it so you will not be able to delete PO, as you will not be able to remove Goods receipt...