The quality and quantity of traffic from every site is down. A decade ago, there were only a handful of sites, and they weren't covered in spam or flashing ads to porn sites. They were clean, easy to navigate, and didn't create a sense of sensory overload just by viewing them.
The web is just too diluted with countless advertising sites that require half a dozen clicks to even read a profile (I'm really talking about Eros)
Places like backpage are easy to navigate, and have the advantage of anonymity in that no passport or drivers licence is held on file to post an ad. Therefore, they appeal to a specific demographic of ladies who will likely be fine with the rate increase.
The advantage to price increases is reduction of customers. So backpages may actually be much more appealing to advertise on, even at $20/post if 80% of people stopped advertising there and individual postings were more visible.
I applaud you for standing up for your personal principles, and for letting a company know that you are leaving. The worst customer is the one who was unhappy and never comes back....but never expresses why they are unhappy to the person capable of changing things. If a company isn't aware of a potential problem, they can't fix it.
However, just like us, websites are welcome to charge whatever they like for the services they provide. It's really a self regulating system. If they price themselves too low, they will be too busy and cluttered. If they price themselves too high, they won't have sufficient clientele to maintain their inflated rates.