I am having the very same problem. Did you find a solution in the meantinme?
Since you have a table listing your options for the value list, if you define an additional field in this table with the price, you can set up a field in your main table with a looked up value setting that will copy the current price into the field.
Here's a demo invoicing file created by Comment and uploaded to a different FileMaker forum that illustrates price lookups from a related table:
great. Thank you so much. My problem solved.