What kind of warranty do you get with the $20 rotors? If you get a warranty of say 50K miles or better, it's probably a good deal as long as they stay in business.
I used to buy rebuilt calipers at Autozone with 50K mile and even Lifetime warranties. I usually need a brake job within 50K miles, so I would bring in my old calipers and exchange them for another set of rebuilt calipers at no cost to me. Just keep your receipts. Oddly enough, they never checked the mileage on my vehicle when I bought the calipers so they would never know what mileage the vehicle had accumulated on those calipers. That did not matter because my brakes only last about 50K miles and I just do the job between 45K and 50K. Of course if you get the lifetime warranty, then mileage is not an issue.
...Rich
