That percentage will probably be even worse than that, since windows aren't at the best angle or position for solar panels.
Just about the only situation it would make economic sense to do this would be if it were almost the same cost of regular glass, and you already had a PV system set up with virtually all the wiring & inverters necessary already in place (these are a significant cost - roughly 50% of a regular PV installation).
Otherwise the cost simply wouldn't make it worthwhile.