So it doesnt matter that the builders dirt has been compacting for 6 years?So that's 900.00 . I guess that would have to be a judgment call based on how much extra they would charge for the compaction in lifts. If you can find it, You could also use a sandy fill as Dave mentioned then use the crushed concrete or #57 for the final 6". Some gravel pits do carry sandy fill.
The one issue I would have with your proposal would be you are leaving the loose layer that the builder dozed out. I would not think it would be worth the effort to remove the sod then leave that extra 12-18". Getting down to the undisturbed loam would be the bare minimum in my opinion. Then once your that deep is it really worth leaving that other 12-18"... ¯\_(ツ)_/¯