A paltry sum that actually isn't worth the effort they put into updating the vanilla game.
I agree. We often stop to appreciate what the mod devs do, but I don't think enough people fully recognize Mojang's generosity, let alone appreciate it.
Minecraft left beta over two years ago. They could have essentially let that be the "finished" game, having fulfilled their promise when you pre-paid, then kept a small team on hand for bug fixes, and moved forward designing "Minecraft 2" or whatever. But they didn't, and they've continued adding quite a bit of content in the meantime. And I have to reiterate, two years of meantime. I mean, one could argue that they just wanted to keep making sales on a winner, but the fact remains that they've never charged us for updates, nickeled and dimed us for new mobs/maps/pets/skin changes/etc, or done anything to take more money from even those of us who bought the game back at the reduced price. There's honestly not that many companies anymore who won't milk a cash cow when they see one.
When you also consider that many of the additions they've made after 1.0 allow for complex adventure/survival/challenge maps (additions which Mojang themselves don't really even use), the amount of content available to play only continues to increase and improve in quality. And this isn't even counting their somewhat friendly attitude towards modding, which I'm sure will still eventually be an officially supported aspect of the game. I also have to point out that Mojang hasn't tried to stop these map makers or modders from making money from their creations, either. A different company might have their own repository where these can be obtained, and lock the game down so that they're the only place you can get them from.
And speaking of making money, someone else pointed out Mojang's policy towards recording video. I'm sure most of you know about the whole debacle with Youtube streamers getting their videos flagged for copyright violations, and having to be part of larger groups to "protect" them, which I would imagine isn't free, and doesn't even always help with the issue. But Mojang lets people literally make a living creating Youtube and Twitch content, the latter of which they built into the game itself. It's publicity for Mojang too, sure, but that argument obviously doesn't mean anything to the majority of game developers who still slap out more copyright violations than new content.
And this is still all completely aside from the ripple effect it makes on the community, whether that's someone developing better people skills from making videos or streaming, better programming skills from making mods, better game development skills from making maps, etc.
So sure, it's still easy to get frustrated if something in the game breaks, or if they don't come through on a feature you've been wanting/expecting. I myself have been guilty of it in the past. But as I've come to see this kind of stuff from Mojang, I think they're a pretty swell company, and I have to recognize that if it were run by some of the other kinds of people we've seen out there, we might very well not even be here talking about mods and modpacks at all.
Now pardon me while I find some anchors to hold up this massive wall of text!