All you have to do is search CB750 on a site such as Cycle Trader to see that a lot more newer model CB750s come up for sale than older models. Almost all of them are noticeably less expensive than even an average SOHC. Often, the older models you do find are in less than great shape, and the majority of people who would be looking for one don't have the mechanical experience to massage an older model bike back to life, much less constantly work at keeping it a daily runner. With advancements such as hydraulic valve lifters, newer bikes can go up to 50,000 miles before needing a major service. Modern CB750s dont need any major engine work, because they make plenty of useable power stock. A 1979 CB750F DOHC made 67hp, whereas a 2003 Nighthawk made 75hp, so I don't tend to agree with your comparison on power between old to new DOHCs. The front forks work great, easily better than any SOHC, that's why Honda lifted them to use on the NT 650 Hawk. The rear shocks and swing arm are a vast improvement as well, with much less flex than any older model.
Now I love an old Honda as much as anyone, I'm not trying to sell them short by any means, but you can not sell a newer CB750 short by saying that it's engine is anything but bulletproof. I know of lot's of mild to wild modded Nighthawks that are perfectly reliable, and ridden as daily drivers every day.
Also, CBR1100 Blackbirds breaking easily with modifications?! Where are you getting that info from? Our shop has a Blackbird with a 1236cc big bore and a Mr. Turbo blower that consistently puts out 215hp at the rear wheel on the Dyno. It has been modded this way for going on 7 years this July, and has never needed anything beyond new clutch plates and tires. Our local fast rider group has 3 Blackbirds that are similarly modded that have stellar records of longevity.
When the 1969 CB750 came out it sold for $1495, which is the equivalent of $9500 in today's money. You could buy a 2006 Nighthawk for $7999. I'd say that was far from a " jack up " in prices.