On the one hand: I have subscription fatigue. I've reached a point at which I'm hestitating to use services because the number of current $10-$20 subscriptions I have no longer fits in L1 cache and it often isn't worth it to me to evict cache lines to figure out what I'm spending --- and so I bounce.
On the other hand, next to the cost of having employees, my rational brain knows I can't possibly be spending real dollars on these things. Put in perspective: every Matasano employee has an Amazon account for which they can buy any book they want ever --- we even have a rule that says "don't worry if someone else bought the book, if you want the book it's yours and you keep it and you can share if you want to but don't worry about it" so that people won't waste time coordinating books instead of just getting what they want and getting back to work. This policy, which we concluded won't cost us "real" money ever, is much more expensive than all my $10-$20 subs ever will be.
But the subscriptions "feel" more expensive.
The answer to the original poster's question is: these things cost $10-$20 because the people running the service feel like they will generate approx. $10-$20 worth of value, and those are the terms under which being in business to generate that kind of value makes sense. In many cases, if the service actually turns out to be worth $3/user, they'll shutter it and pick something better to work on.