Amazing the cost range. I think I'm in a good position to keep it way on the cheap end of things. I've decided to use my parts 1981 CB750F as the base (after I try getting it titled.) It's completely disassembled but basically all there. I have a Bridgeport style mill, lathe, and MIG welder in my garage, so make custom parts is no issue.
I think it would be best for me to buy a used sidecar instead of building on from scratch for my first setup. So over the winter I'll keep my eyes open and hopefully score something decent around $1000-$1500.
First setup would be just bolting on the sidecar. Then I'd work on building a leading link setup/car tires/sidecar linked brake etc.