I think it would be a good selling point, but wouldn't necessarily raise the value of the house by the amount of the investment. i.e. If the house is worth exactly 400,000 today, and you work for the next 24 hours installing a $2k central vac system, then tomorrow it's not going to be worth 402,000. I'd think of it more as something where someone says, "wowwww, and it's got central vac?!" and would help distinguish it from other similar houses as one with more features.
However, if I gutted my house and happened across all the components I'd need for central vac at an auction for $100, would I bother to install it? No. I'd rather deal with the simplicity of plugging in a quality vacuum into any wall outlet in a room, and the ability to wheel it from room to room than have to deal with arranging furniture so that I can easily get to the central vac outlet, and having to drag a hose all over the house.
20 years ago, central vac had one incredible benefit: dust wasn't pushed back into the air in the living space. Today, quality vacuums make that feature moot.
edit: No, I've changed my mind after checking on a few central vacuum sites. I think I'd definitely install it if I had the chance.
For what it's worth, is the price you're quoting the installed price, i.e. someone else installs it for you? Are you paying someone else for doing all the work on your gutted home? It seems that if you're in the business of flipping homes, there's a higher profit margin when you do a lot of the labor yourself. The level of difficulty of installation looks about like something I'd delegate to one of my children to install (15 and 17 years old.) The cost of materials appears to be quite a bit lower than the price you were quoting.
Questions I'd have about these systems: I've noticed in the diagrams of installations that occasionally, they use t's. I've clogged my shop vac on many occasions, particularly where the hose enters the vacuum and material has to make a turn. Wtf do you do if you get a clog in your central vac system?? But, then again, the materials I'm cleaning up with a shop vac aren't typically what you'd be cleaning with a central vac.