It does take a lot of work to make tilesets, but a tileset isn't a one-time make it and sell it thing. They can sell the same tileset to as many people as they want. Suddenly a month's worth of work could equate to three month's worth of pay.
On the consumer side of things, $20 is a lot for a group mostly comprised of young hobbyists. Even as an adult with a steady paycheck I had a hard time justifying my purchase. If someone was really serious about going commercial, it'd be an alright deal. However, you'd still run into the problem of not having much variety to work with.
I dunno. I guess it's ultimately a personal decision whether or not you think something is worth a certain amount of money. It's definitely a well made set.
Oh, and thanks for the compliment by the way.