Getting the exhaust quiet is not hard like Andy said, just some hospital grade mufflers. 5 years ago I think they were around $900 a piece.
Getting the air in and out without letting the sound out is the trick.
I built the room myself and it’s pretty quiet. The only time the neighbors have told me they could head it was when we had a 2200HP engine on it.
The cost of the room and the support equipment IE Fans, pumps, plumbing, electrical, VFD’s and storage tanks ran me another 25K after buying the dyno. Again, that was me (and a few friends) putting it together.