You don't need any scripts nor any script trigger. You do need this relationship:
Estimate Data>------Part catalog
Estimate Data::_fkPartID = Part Catalog::__pkPartID
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
With that relationship in place, when you senter or select a part ID in _fkPartID, a Unit price field in Estimate Data can be set up to auto-enter the unit price from the corresponding record in part catalog.
Find the Unit Price field defined in the Estimate Data table in Manage | database | Fields. Double click it to open Field Options. Use the Looked Up value option on the Auto-Enter tab to copy over the unit price from the unit price field in Part catalog.
If you are using Filemaker 12 or Filemaker 11, the Invoicing starter solution uses this type of relationship and auto-enter setting to copy over unit prices to a table of line items (or Invoice Data in Filemaker 12) when a product ID is selected.
Thank you for your timely response to my Posting.
I'll give this a try and see how it goes.