Going market rate for 500 pieces of 5.7 once shot brass is 50. I imagine 5.7 reloading will become more popular over time but nothing compared to most calibers. If you actually know someone who reloads 5.7 give it to them or stick em in a ammo box and keep em.
I've sold 9mm brass for 4 cents a piece (1000 pieces of brass) and that seems to be the general area for common brass. Stuff like 5.7, 6.5 Grendel, 338 Lapua will get a bit more, to a lot more (338 Lapua brass is ridiculously expensive).