I used to work at Nordstrom and bought a some there during the Anniversary Sale. They have held up extremely well (10+ years). Recently, I found some of the same type at an estate sale. Score! I have purchased less expensive bags, but they have not held up. Also, washing bras and other small items this way makes the washed items last longer, which quickly pays back the initial cost of the lingerie bags and them some.
Goblinchief, if you hook 'em, bag 'em, and line dry them, you will easily double their life expectancy. Those babies ain't cheap!