It depends on how you run your business. The basic approach is to define a looked up value field option to copy the price from the pricing table. That way updates to your pricing table do not modify the prices used in existing records. Once you have that approach working, you can also consider whether a more sophisticated pricing system where you can schedule price changes by effectivity dates (and have a history of past price changes) is of value to you.
Here's a simple Invoicing Demo file created by Comment and uploaded to a diffeent FileMaker forum that illustrates using simple looked up value settings in a products table to move prices into the invoice's line items records: