Build a Report Definition Rule that retrieves the price of an item based on its item ID. The Report will require one or more Parameters that you use as filter criteria for the Report.
Build a Data Page Rule that uses the Report you built in Step 1 to retrieve the desired price. The Data Page will require the same Parameter(s) passed into the Report. You will have to pass the same Parameters to the Report.
Build a Data Transform Rule that Copies the Price from the Data Page to the Price Property.
Configure the Client Event "On Change" for the Cell in the Layout that contains the original Item Drop-Down to "Refresh the Section" and call the new Data Transform built in Step 3.