I don't know where you are but the very best prices that I've ever found for for Birkenstocks are at the Birkenstock outlets. There used to be only two in the country - Gilroy, CA (south of San Jose) and Orlando. I always stop at the one in Orlando and I rarely pay more than $20 for a pair. Of course they may be silver or orange. I got a great pair of of black leather ones ($150 retail) for $20 once.
I think I heard there was supposed to be a third outlet opening in the Northeast but I don't know anything about it.