Use Lookup Column in Formula

Use Lookup Column in Formula

How can I use a lookup column as a part of a formula or what would be an alternative to this?

What I am trying to do:

- I have products and product prices stored in DB1
- In a sales DB I am using a lookup column to get product prices from DB1

Now I want to automatically calculate product prices (lookup column) * amount of products sold in the sales DB but for some reason I cannot use any lookup columns as a part of formulas.

Any help would be much appreciated...

Thanks in advance!