Can you really rent them for $56? Is that through the guide service? That is a very good deal.
People have experiences all over the map with rental boots. But most of my observations are with renting plastics, which are pretty inflexible. You HAVE to get the fit right with those, or you'll suffer. The Nepals have got a bit more flexibility. You may just want to ask the guide company (if that's who is renting them) - what are people's experiences like with renting these boots? You could also try them on - it's possible they're just not the right shape for your foot, in which case it would be a disaster to rent them.
You can definitely find them used for a couple hundred bucks. But you'd probably need to order them from e-bay, and then you're stuck with them, even if you get the size wrong or whatever. $500 is a lot of cash money, but maybe you could use them on your day hikes this winter, break them in really well, and sell them if you're not going to use them again? Or is it possible you'll want to keep them? They're great boots and should last you quite some time, probably 10 yrs or more. $50/year is not very much money.
My opinion: you'll have enough to be stressed out about for Rainier. It would be nice to NOT have to worry about whether or not the boots you rent are going to completely ruin your trip.