Expensive if custom made...and expensive to install...We need some full gearsets, some straight cut syncro mesh, and straight cut constant mesh...That would be the best option probably...and with other cars they don't seem to get too ridiculously priced (especially the simple design of the constant mesh)...
If any of these Turbo maniacs do get fored rods/pistons and start pushing 300+whp, stronger gears and case reinforcements will be needed...I doubt there will ever be a market for it, but they could be very beneficial...