Some Crazyness I am trying

Okay so here it is.....
I have a Database that is in Access. It is a inventory database mainly but I also want it to double as a means of keeping track of costs of production for each product made.

Each product has special component parts. If I included each of these my form would be 2 pages long and I dont want that. So I have a table that has each product and each product's component costs. These are then added together in a total colum. This is actually being done using a linked Excel workbook. But I might make it a regular table if need be.

So what I want is in the form a combo box that drops down and shows you the Product name, but when selected it gives you the total price.

I then need this total price to be added together with the other prices that are entered into the other boxes on the form. I had an expression built for this and it worked great, but now that the compnent price is a combo box it no longer works.

If you could give me any ideas on how to code this or what ever please let me know