Standard GP functionality calls for the Originating Invoice Cost from the Item Vendor window to automatically populate on a new PO.
*click image to enlarge
In a GP 2013 implementation, I was unable to get the cost to default on the PO. After checking and re-checking a number of settings, I discovered that the 'POP Cost Defaulter' was turned on in PSTL (Professional Services Tools Library). POP Cost Defaulter disables the standard functionality and allows you to default the PO cost from either the Standard or Current Cost fields in the Item Card. Apparently, someone had enabled this feature and selected 'Standard Cost' as the default option, even though we're not using Standard Cost in this environment.
Thanks to my colleague Chris Liley of IBIS, Inc. for helping me solve this mystery!