I believe you would want to add this as the below; and I have attached the Purchase Orders and Inventory Transfer Template Variables for your reference:
[View:/cfs-file/__key/communityserver-discussions-components-files/106/Purchase-Orders-and-Inventory-Transfers-Template-Variables.docx]
Price at which the store sells the item in the specified purchase order entry.
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NOTE: Mainstream support for Microsoft Dynamics RMS 2.0 ended on July 10, 2016; and extended support ends on 7/13/2021:
community.dynamics.com/.../microsoft-dynamics-rms-2-0-mainstream-support-discontinuation-july-10-2016
support.microsoft.com/.../search

This is less than 6 months from now; and you can continue using RMS after this date but technical support will not be available.
I would recommend checking with your partner in regard to what they may recommend as a replacement system after the extended support end date.