Appraisal may also establish stumpage value as if unconstructed roads or other developments needed by the purchaser for removal of the timber were in place. When timber is appraised and sold on such basis, purchaser credit for road construction, not to exceed the estimated construction cost of such roads or other developments specified in the timber sale contract, shall, when such construction is accomplished by purchaser, be deducted from stumpage payments made by or due from purchaser under the timber sale contract for other than minimum stumpage rates and required deposits for slash disposal and road maintenance. As used in this section estimated construction costs means the total cost of constructing all permanent roads specified in the timber sale contract, estimated as if construction is to be accomplished by an independent contractor who is not the timber purchaser. In determining the purchaser credit amount applicable against timber payments, the estimated construction cost may be reduced for the effect of differences in applicable wage rates.