GoGo is quite aggressive about variable pricing. An all-day pass is $16 if you buy it ahead of time, but if you buy it on the plane it can cost up to $50. They also do split testing of their prices and discriminate based on the user agent.
I just realized I never realized they were different, since the name is so similar.
You also get access to Delta lounges (when flying Delta) and Amex Centurion and Air clubs at airports that have them.
If you travel regularly it's worth it.