Bingo, bro.  The cost of electricity or whether the mining equipment is already paid off only makes a small difference--and it's an unlikely scenario in most cases anyway that power is going to be free and mining has produced enough profit to pay for all the hardware in full.  And if you're talking about a solar-powered mining farm, that just isn't going to happen if people don't pay fees (or very small ones).

I'll admit I'm not expert in mining.  I know very little about it, in fact.  What I'm wondering is what would happen if bitcoin's price reached a level that was so insane that 1 satoshi was something like $5 or even higher.  Would that not screw up the entire bitcoin system?  There would be no way to send small amounts of bitcoin or very exact amounts, much less pay a fee to the miners.  Am I missing something in that hypothetical case other than the fact that 1 satoshi will probably never be worth that much?