My thoughts on the subject.....
First, our typical contractor bid for irrigation now runs from $1400-$1600 per sprinkler, up quite a bit from a few years ago. That said, the typical pricing pattern in the industry (and maybe all construction) is to hold steady, especially in recession years, and then jump 15+% when the economy improves. I think there is some of that, even in irrigation where demand was fairly constant. The other part was, as mentioned, the increase in oil prices, which affects plastic prices and transportation, although it never seems to go back down when gas drops back to under $2 per gallon, LOL, Not.
We estimate the number of sprinklers at 10-14 sprinklers per acre, with 11-12 being most typical with average spacing. As Mike Nuzzo alludes, sprinkler spacing has been getting tighter, from an average of 75-80 feet years ago to 55-65 feet now on windy sites and under the theory that closer spacing actually saves water to some extent by providing more even coverage, i.e., supers don't have to run longer to adequately catch the outer radius areas of each sprinkler, thus overwatering the center of its pattern. Like Tom D, I have asked for data. Toro and Rainbird have stated you can save 23% of water, which is independently tested and verified. I believe you actually can save maybe half of that 23% in real-world conditions, but the data I have seen from old and new systems at the same course, it doesn't often happen that way. It is hard to use more water using an old system of perhaps 1600 GPM capacity over a new one with 2500-3000 GPM capacity.
These type of systems are often regulated or strongly suggested as "best management practices" (which some courses follow in an attempt to minimize stricter regulations in a given area)
Anyway, my take is that it is not even the number of sprinklers used, as it is the perceived need to water every acre of golf course every night, if desired. And, in the last dozen years, to water it in 6 hours, not the 7-8 of years past. That is what drives the GPM capacity so far upward. 6 hour water windows were originally started at high end and resort courses, where they wanted to have maintenance personnel off the course before the first golfers to enhance the golf experience. The best cut comes after the turf dries out somewhat, so to be done mowing at 6AM, watering must stop at 3AM or so and can't start until about 9PM if you don't want late evening golfers in summer to see sprinklers running then, either. So, pump stations and mainline pipes have both gotten extraordinarily larger. A typical mainline exiting the pump station might have been 12", now its sometimes 24" and never less than 16", in my experience. The 6 vs 8 hour water window really drives up cost, and at most courses, playing follow the leader, I wonder if it impacts the golf experience enough to really be worth it.
There used to be an oft used option to design a system to water every other night. If using the checkbook system, watering only when field capacity is low, you are probably watering every 2-4 nights most times of the year. In the "old days" when it came to August hot days, they simply ran the system 10 hours or more, but now they seem to be designed to never have to run past 3AM. If you ran the math, losing a few hours of play per day in August (and when it's 100 deg., you probably are losing most of it anyway) may not cost more revenue than the cost of the system. Of course, in a 1-3% interest rate environment, it's not really the $1.75-2 Mil total cost of the system, its the annual debt payment that really matters, and it's a good time to spend more now to save water later in some cases. (I'm just not sure it pays back everywhere) But to be fair, there are Mercedes type irrigation designers who put in everything, and there are GMC designers who don't insist on every bell, whistle, and GPM of capacity.
I asked a Texas superintendent (around even longer than I have been) what would be the problem with extending the irrigation window on those few really hot days, like they typically did in the old days. After thought, there wasn't really a big technical problem (although theory does say to finish water before hot sun). Then, he said the old guys might strip down and run through the sprinklers naked.....and no one wants to see that!
As with most discussions here, we tend to focus on top end courses, that make up a small percentage of courses. And, yes, at those type courses (and the threshold is truly dropping for courses that would accept some season browning in outer play areas) members sometimes direct the super to water more than is technically necessary for turf health. Many supers have adopted irrigating a few tenths every night, because someone complains that the course plays differently from day to day (I doubt most could really notice.) That same idea scotches the idea of designing for every other night watering to reduce capacity and water use, even if that is really better for the turf. So, yes, in some cases, the fox runs the hen house and I bet most supers would be on board with deep, infrequent watering if not for outside pressure.
In reality, most courses struggle to irrigate enough, not have problems of watering too much. However, I agree the cost of the new irrigation system seems astronomical, even if part of that is my old guy, gum used to be a nickle self, LOL, NOT!