Imagine, that you want to change building materials for other goods. What is price? How much wood = how much goods?
Answer is, that 2 wood = 1 luxury resource.

Why?
- 1 worker is enough to gain 1 wood
- to gain 1 luxury resource, you need 2 workers (more or less)

So - 1 luxury resource = 2 wood = 8 gold (2 citizens earn for you 8 gold per hour, 2 wood or 1 luxury resources)