Yes Dox, I'm like a squirrel, I generally keep my bikes. Had my Z900 since '76, the 955 is twenty-six years old, the SS is even older !! My garage is too small now to get anything else in, without getting rid of anything, which I can't bring myself to do !!
When I built the first old nail I never intended to build another, but ............ what can I say !

It may well be I enjoy the actual "building" more than the taking them out, don't get me wrong I absolutely love going out for a ride on them. I just need more room as I'd like to do a Ducati Pierbon next, or something similar !
In terms of cost, although the bikes "look" similar the parts are in the main unique to each bike so other than the big bits like the engines, primary drives, gearboxes & front wheel/disc, everything else is a one-off. So the machining costs would still be the same for those parts I've had done outside of my garage anyway.
Also, if you built two, once they were finished I'd most likely feel duty bound to sell at least one of them & you would NEVER get back what you had invested into each bike & that's without even starting to add a cost to your time building said bike. That's a huge amount of money to "give away" although have two of everything would stop the kids arguing when I die !!
Being semi-retired you need to have an interest/project to keep your mind occupied otherwise my wife would have me decorating all the time, which doesn't bear thinking about !!
