The official mods cost money because of all the work Bethesda had to do, what with making the content, removing said content from the actual game, and then putting it into mod form so it can be re-inserted back into the game.
I mean, let's say you're a mason. You have to build a wall for Mr. Crabb and his family, out of about 100 bricks, and they give you $200 for it. But then, you carefully remove some of the bricks, fix them up so they can be put back into the wall without too much trouble, and sell them back to the Crabbs at $5 a piece. See, if you had just put up the wall as it was, it would've been much easier, but the added workload of brick removal should obviously not go uncompensated.
And since Mr. Crabb is far too stupid, not to mention in awe of the realistic reflection of the bricks' surface, to use the tools that came with the wall to do it himself, or even find someone who's willing to fill out the holes in the wall for him, he'll pay for the costly bricks without giving it another thought. And so, Mr. Crabb has become a victim of the business method known as profit maximization, but since Mr. Crabb is too busy staring at his realistic wall to look up those difficult words, he'll never find out what's going on, or how badly he's been fucked.
The end.
