i think it all depends on what the used jeans in question retail for. $20 for used Levi's is borderline-absurd, but $20 for jeans that retail for upwards of $200 (i.e. j brands) isn't all that unreasonable.
then again, maybe i'm just a label whore

i also thrift in nyc where everything's overpriced, resale not excluded. what does secondhand high end denim cost elsewhere? am i getting totally ripped off?