Fields In One Table, Depending On Another

Right, my first access database is coming along well, with the help of some people on here, but I have another question. The database is designed to track jobs in our small workshop.

We have a 'price matrix', that has our regular customers on the left, the material type long the top ( mild steel, brass, etc), and a cost in each area for each of four types of standard job, so i suppose its a sort of a 'cube' rather than a matrix, except not all customers and materials have an agreed price for each of the standard jobs (there are only four of them, a maximum of 3 per customer)


Now, my database revolves around a 'main' table, with a unique order ID, customer name (from lookup table ), and other details. Initially, I have created another 'cost matrix' table, with a primary key, customer ID (based on the same lookup table), material type and job type (both lookups), and price fields. What I want to do is that my guys can select material and job type, and the cost will be recorded somewhere. Now, I understand that you shouldn't have the same data twice anywhere in the database, so I figured of I am recording the material and job type in question on the main table, and given the customer ID is recorded as well, I should be able to reference this to my cost matrix table and then be able to populate a field on the main form, query against it on reports that sort of thing. However, there is a field on my main table for cost adjustments ( say we work 24 / 7 on a rush job), which is manually entered on a job by job basis, so should I record the total job cost in my main table, or always calculate it when needed using a unit cost + adjustment type of calculation?

Also, how do I populate the field on the main form that the cost should be 'x' from the'cost matrix' table, given the values of the customer, material type and job type fileds?

All help greatfully received. :)