Cash is always and issue but the SRM solution is not that straight forward. You need a timing chain on the captain and stoker cranks and typically a triple. I suppose one could be motivated and have some special work done on the SRMs to make it work.

I think the Ibike is great or hopefully a pedal based system that will still be expensive but will also work on the road bikes.

And if I were going to use a crank based system and figure out a way to "make it work", I would use the Quarq. We have this technology on road bikes and it works well for less money than an SRM.