Invoicing, Quoting, and Purchase Order systems all have the same basic set of tables and relationships and you can see these in the starter solution files that come with FileMaker:
Customers---<Invoices (or Quotes or Purchase Orders) ----<LineItems>---------Products
Do you have such a set of relationships.
Generally, static information, such as a base rate per square millimetre is stored in products. (Specify a wooden sign and get a rate for wood, specify aluminium and get a different base rate, etc.
This info is looked up into LIneItems where quantity and/or dimension fields use the looked up rate to compute a price.
Please note that you are not limited to a single formula for computing a price, different types of items can use different methods for computing a price for that item in the line items table. Your labor cost might be a line item record that computes Hours X Hourly Rate while your sign is computed as Length X Width X RatePerSqmm
Thanks guys, this helped get my head round it a bit better.