Would this by any chance by for a sales invoice? If so there's a starter solution that you can look at for how an invoicing system might be set up and I'll include a download link at the end of this post for a simplified demo file that is easier to figure out as it's stripped down to just the basics.
Normally, you list your items and their unit prices in a separate Products table and link it in a relationship to your layout's table. Then, looked up value settings on a unit price field in your layout's table can copy the unit price from your Products table when a matching productID is entered or selected in the layout's productID field.
This Demo was created by Comment and uploaded to a different Filemaker Online forum: