Toki Tori: The Cheapest Game on Steam

Technically not the cheapest unless "free" isn't considered to count as "cheap". There are free games on Steam for Linux, and I'm quite fond of one of them:
Sunrider: Mask of Arcadius is available for free on Steam for Linux. It's not an open source game, it was developed with some sort of crowdfunding money and then released for free. And while it's partly an interactive novel thingie, it also has turn-based tactical spaceship combat, so it's a real game even if you don't consider novel thingies to be real games. And really, the combat is quite fun and challenging. You command a carrier and a little fleet of space mecha and eventually, optionally, some secondary ships. Plus, you get "prize money", as it were, from the space battles, and you can buy quite varied ship and mech upgrades and a few other goodies. There's a nice mix of weapons which behave quite differently from one another. Also, successful battles give you "command points" which allow you to go all space opera by giving various commands to do "above and beyond" sort of stuff, whether simply fighting more aggressively ("Full Forward!") or unleashing the wrath of the experimental Vanguard Cannon. Even though there's no real animation, there's something satisfying about watching a series of enemy ships go boom as the Vanguard Cannon's beam bisects them.
Warning: The art is seriously in the "voluptuous anime girl" style, while the plot mixes anime harem with classic space opera.

"Mask of Arcadius" is the second piece in a planned three installments. Third is expected this year and will also be free, although I'm sure they'd be happy of any support when they run the crowdfunding for it.

Toki Tori 2+ the sequel is very different type of game (puzzle-platformer "metroidvania style" ) and very fun.

Great vid! We really need more kids' games to be honest, I'm sick of just seeing different shades of grey in all the AAA games that come out (the real world doesn't look like that, nor do we need to imitate it!).

Just one inaccuracy I noticed. You said at one point that some aspects of it are not mobile-like, and mentioned the timer and the 4 or 5 stars that appear at the end of a level. Actually, this is a defining characteristic of many, many casual mobile games and goes well with their short levels that you can typically finish in about 5 minutes. It was pioneered by Angry Birds, which gives you a 3-star rating depending on how cleanly and efficiently (with as few birds as possible) you knocked down a structure. So that's very much mobile game inspired, if anything. I wouldn't be surprised if it came out on mobile said, as you mentioned.

Another great free game without any form of micro-transactions is You Have to Win the Game, which is a metroidvania-esque precision platformer without combat.

The Tribes pack isn't DLC for Toki Tori, they're different games bundled together. EDGE is really fun in that bundle. I didn't expect to love a puzzle platformer so much.