Cost me two x $25 garage sale junk bicycles and $20 for two new chains bought on ebay.
The epoxy, fiberglass and plywood I had sitting around (because I build wooden boats too) but that is worth perhaps $25. So, for $95 I have a functional homemade xtracycle-clone, or what-cha-call-it bicycle.
Built in spare time sporadically over several months of time, maybe 20 manhours.
I suspect that some will question the strength of the epoxy, and fiberglass roving, used for the joint connections. My opinion, based on a lot of experience using these materials, (and based on the road testing so far with this specific experiment) is that they are plenty strong enough. Epoxy has spectacular adhesion, and the glass fiber has a very high tensile strength.
Also, the plywood is 1/8" thick, which when bent, takes on a surprising rigidity, and light weight qualities.