The most common answer is "It Depends."
On a things like:
Whether the construction is all timberframed? (vs. stickframed hybrid)
How much vaulted space is in the design?
What sorts of architectural details and millwork are in the design?
How elaborate the joinery is?
A commonly used figure is a minimum premium of 15-25%, but if high-end features are incorporated it could go to 100% or more. It's up to you --- remember, contrary to popular belief, timber framed homes don't require granite counters, Garland stoves and media rooms.