We used triple-junction solar cells on our satellite. If I remember right the cells ran somewhere around $100-$200 per square inch. I don't remember the efficie

We used triple-junction solar cells on our satellite. If I remember right the cells ran somewhere around $100-$200 per square inch. I don't remember the efficiency, but they were left-over dumped cells that were donated to us by some group at NASA because they "expired" (they're kept under a nitrogen purge because they oxidize slowly and these had a leak, if memory serves, still worked great though) and a few were broken as they are more fragile than glass and are about as thin as a sheet of paper. They glow bright red when you forward bias them which you can use to visually detect dead portions of the cell/damage.