There's also the question of how much of the electricity is paid for. E.g. how many miners are mining on "free" electricity at their parents house, or have access to rack space with "free" electricity (our racks at work come with a certain amount of power, for example; someone putting a Bitcoin mining rig in there would not increase our costs as long as the power doesn't exceed certain limits), or has a miner in a quiet corner at work or similar?

Or any number of other ways of getting free / outright stealing electricity.

maybe they don't have the money to buy them because it's been all spent for mining gear?

that would require a $500 dollar cost of electricity per coin, however in reality the $500 is made up of the investment in the hardware and electricity. You could not just switch off the machine and have $500 to spend on coins, you would need to sell the hardware. of course in most cases selling the hardware for USD and spedning it on BTC will generate more BTC than the hardware will mine.