They only make so many and if the demand is more than the supply that they want to create the value goes up and so does the price they charge...
Look at Eagle E-type Jaguars. Same thing, different marque.
I think you can throw a lot of money at a project 911 and get something that is probably more unique to you and at as high a quality as a Singer but you would have to spend a lot more of your personal time as the Project manager / General Contractor, you would have to go to multiple shops for mechanical, body, interior, engine/gearbox. And, like building a computer, the great pieces might not work as well in company as lesser but more harmonious pieces do. Singer has tested the pieces so they work great together. That's worth a lot as is the time to find all of that out.
They are also like little jewels when it's finished. You might get a better engine but it probably won't be finished nearly as well as the one below.
Past-President, Porsche Club of America - Upper Canada Region