Mostafa, you forgot the ancilliary costs too.
The weight of the bang seat, instruments, cabling, coffee holders, ingress/egress, extra canopy etc. Call it an extra 300-400kg at least.
The fuel tank and feeds would reduce the gain but not too much.
One neat solution, Swordfish-style, would be to retain the cockpit space but denuded of all the ancilliaries, and put a fuel tank there. If, later on, you wanted extra space for a passenger or whatever, then you could remove the tank.
Didn't the Saab Lansen have the capability of carrying two passengers by replacing the bang seat in a ferry mode?
