How much should bridesmaids spend on dresses?

I love my best friend, but she has picked bridesmaids who are 18-20 years old. They are all students or unemployed. Her dress cost $170 through some internet hot hot hot deal from china. Which is awesome, but the dresses she picked for the bridesmaids are from an expensive bridal store and cost as much as her wedding dress. I think that's unfair. I would like to talk to her about it but I would like to have an idea of an expectation. Is there some sort of etiquette or percentage that is universally acceptable?