If solar becomes 100x cheaper than today's energy sources, but bitcoin mining still pays a large reward, bitcoin miners will just use 100x more energy, right? They're in a race with each other, it doesn't make sense to leave any extra hashrate capacity on the table since if you don't use it, somebody else will and they'll get more of the block rewards.

Maybe that's fine, since solar is still zero-emission, but surely there's still some cost to producing that energy (such as the space taken up by the solar panels, the cost to make and replace them, etc). Otherwise it'd be free, not just cheap.