I'm surprised they don't experiment a bit more to find out....
There's a very interesting progression on how pricing these kinds of packages works (see: history of cell phone pricing):
> step one: charge what you need to cover your fixed costs given your initial subscriber base
> step two: once your fixed costs are covered (you have a required minimum number of subscribers), you can increase overall revenue by then reducing what you charge more slowly than your subscriber base increases, since incremental costs are a small part of your total charge
> step three: you now have so many subscribers that your fixed costs now are a small fraction of total revenue, and it's all about finding the "sweet spot" between size of subscriber base and what you charge them.