Prove it. People have already debunked the "time" part, so prove the cost part. I don't think you can.
Courses would still have to maintain tees, fairways, and greens. And bunkers. Sure, they'd have to maintain a little bit less rough, and a teeny bit less fairway, but the biggest costs are tees, greens, and bunkers. The rest is just mowed with a guy pulling some blades.
Land cost, for one. It takes less land to build a 6000 yard course than it does to build a 7500 yard course. Water cost is another factor, as it takes more water to water 7500 yards than 6000 yards. Fairways and rough get fertilized, so that cost goes down. Fairways get mowed, so that cost goes down. Mowing takes longer and requires more crew and more equipment to get it done in the time they have to do it without interfering with play. Come to think of it, rough also get mowed so there is that as well.
As for the so-called time debunking, shorter shots do not go as far off-line, at a given degree of miss. How much time do we waste watching a couple of powerhouses play where every blasted tee shot has to be tracked down in the outer reaches of the rough. Or beyond. The reason the guys who are near the top in driving accuracy are rarely the ones near the top in driving distance is not that they are intrinsically more accurate, it is that with the same degree of miss they are still in the fairway where the guy who it it 20 yards further is in the rough.
It is a little simplistic to take the difference in length and divide it by the speed of a golf cart. I play damn fast and one of the reason is that I rarely hit it so far as to get myself in serious trouble.