This is a typical invoicing set up. FileMaker 11 comes with an invoices starter solution you can examine to get ideas and here's an even simpler demo file you can look at that was created by Comment: http://fmforums.com/forum/showpost.php?post/309136/
Both examples use a related table of product prices which are copied into your LineItems table via FileMaker's Looked Up Value field option.
Using your values for an example, if you enter the ID for Product 1, 9.95 is looked up. If you enter the ID for Product 2, 6.00 is looked up and so forth. This allows you to change prices and add or remove products simply by editing the records in this related table and you can compute the line item cost with a simple formula: QuantityField * LookedUpUnitPrice.