The 50.00 is per server... not site. So if you are running 20 sites off one server.... you will be covered.
One thing I noticed about your post. You are saying that a 10 minute surge in BW is effecting your BW price each month. I really dont see how that could be, even if it was a 10 minute spike once a day. If it is, you really need to have a serious talk with your hosting company. Most all go my hour avarage so I 10 minute spike, once a day should not effect your overall bill... not much to speak of anyways. I just dont see it.
The hacking sites that are popular update at midnight est. Chances of you making the big sites every day are slim. We hit them once or twice a month.
BTW... there is a reason why proxypass is more then the others... it actually works everytime. They have great 24/7 support and they back their results. You can try the others, but you will end up and proxypass sooner or later.