When setting up the PO, there is a "Show costs when printing" tickbox that allows you to choose whether to show or hide costs on PO's and work orders when sending them.

The thinking here is that if in many cases it isn't relevant to a supplier/trade what you've budgeted for the items, hence by default we don't show the cost on a purchase order.

However when referring to an agreed rate or a trade quote it can be really handy to show the supplier/trade the rate they quoted you and confirm you're both on the same page, hence having the option to show the costs also.