A really difficult subject to make any sense out of, bikes are so many different things to so many different owners.
I dont think there is any doubt that the cheapest way is to buy a fully restored bike, provided you can find one with a known history so you can have faith in the rebuild, unfortunately that is almost impossible. See what I mean by it all being hard to make any sense out of !
One thing we should be 100% clear on is the quality of a bike is directly related to money, no ifs or buts, its all about money. You can not do a quality restoration cheap.
If you want a 100% correct immaculate bike you have to be prepared to pay. If you are flexible in what you want then anything is possible. Most of us fall somewhere in the middle of all that, ranging from concourse to something that goes. My bikes are not immaculate, I dont enter in shows or put them on display, my bikes are for riding and they must start, run and ride properly. However I do try to have them presentable, I keep them clean, mimimise oil leaks and try to have them look as they did in their day.
As for what to buy then that depends on what you want, how much you can afford and whats available where you live (or close by). A bit over a year ago I wanted to buy a swingarm model B33, I searched for months for a unrestored one but simply could not find one so I bought a restored one. I got it home and totally dismantled it and started again with the mechanical side of it. It was a nice looking bike but was a mess mechanically. Thing is I bought it because it was the model I wanted, and it was located only 450 k's away, I had narrowed my options by wanting a specific model close enough to me to be able to go and collect it.
As for money, anybody that is involved with old bikes that thinks they are going to make money from them is living in dream land.
So I guess this all brings it down to why we ride them, it would be a lot easier to buy a new modern one. Secretly I think we all like it when something goes wrong and we are able to diagnose and repair the fault, thats what makes it interesting. And that in itself is one of the good things about our oldies, when they do break down we can fix them unlike moderns that are electrically way too complex for the average person to fix.