Hi Catherine,
The FSC premiums vary greatly depending on wood product, species, grade, dimension, etc. The lumber industry in general seems to be volatile right now as well, the general costs of even non-FSC lumber has been trending upwards due to recent demand, forest fires, and Covid. Depending on what types of wood products you’re looking at, I think reaching out to your supplier/vendor is the best option. I’ve seen everywhere from a 0% FSC premium for specialty products, all the way up to a 35%-40% premium for commodity 2x material.
FSC also has a handy wood supplier finder: https://sourcing.climatesmartwood.org/