Ben,
Irrigation life span varies a lot, depending on materials used and design. The old standard was Class 200 PVC, but many used Class 160 to save money up front, at the expense of life span. Now the standard is HDPE, a bit more expensive, but almost indestructible. Of course, fittings are always the weakest link on PVC pipe, and they range in life span, too.
The other factor is the original (or in the case of head additions over the years) flow velocity. If designed around 4-5 feet/second, pressure at the joints is much less and a PVC system can last 25 years or more. If the system has velocities of 5-7, after 15-20 years, you get into a few breaks a week. At some point, the cost of fixing is at least as much as the cost of debt on a new system.
To keep costs near a million, the only ways to do it are to reduce both bells and whistles (i.e. a degree of control flexibility) and pipe size, usually done by keeping the water window at the traditional 8-10 hours. The modern super is asked a lot, and some of it is keeping their operations away from golfers at more and more clubs (i.e., mow very early or even at night) In response, many new systems aim to water in 6 hours or less, so watering allows players to play to dark and stops hours before the mowers need to start running in the morning. Nice ideas, but does increase pump and pipe size substantially.
And, on the hottest days, it means you may be watering periphery areas during the day to keep up. But, thousands of courses have managed that, so maybe standards are getting too high. I asked an old super what the risk of watering on hot afternoons was and he replied, "Old guys getting naked and running through the sprinklers."
I stand corrected, maybe watering in six hours is worth going into hundreds of thousands of extra debt to avoid that!