There's not a lot of information out there on timber basis. Most of it is dedicated to people who are in the "timber business". In doing a little quick research, it appears you can take a little less than 50% of the basis of the land where the timber is located. 

Quick example: Let's say you bought a house and 30 acres for $600,000.

If house structure is worth $300,000 that leaves $300,000 for the land and any other structures.

If you have 10 acres of pasture, fencing, and barn, maybe that's worth another $200,000.

That leaves you with 20 acres of logging land at $100,000. If you use 60% for the land and 40% for the timber, that would be $40,000 for the timber.

I found this timber basis calculator from the Timber Tax Institute. Look over the example and then you click "Determine your basis now".

https://timbertax.org/getstarted/basis/purchased/