What makes a good short manga? Most manga are written with the idea in mind to make them go on for quite a while, or even with a full set plot in mind, may go on for several volumes. In this case, Life is Money spans only 3 volumes, which is, in my opinion, the perfect length for this story.

Plot: Meguru needs 100 million yen for his sick sisters heart transplant. After being scammed by a group of people and outcast by people who believe he stole the money, he's desperate. So desperate that he listens to the sly words of a bar patron who tells him of a high risk game that can earn him 100 million yen. With his deathly ill sister in mind (and more tricks from the bar patron) he agrees.
What follows is a game similar to one you could find in Liar Game, though instead of a deadly debt, there is literal life on the line. It's a mental game, if a player gets to stressed out, or breaks a rule, he'll be trapped in a mental prison and die. There are also other elements, such as cutting off a players sense to add to the challenge of the game.
It's a survival manga, not much more too it. But what follows is more interesting, an event comparable to the stanford prison experiment. At first it seems simple, all they have to do is ignore each other. But quickly the rules change after one of the player hogs the only food they were given and says he'll only hand it over if they agree to give him some of their winnings.
The plot progression is not the most interesting part of the manga. You could almost call it predictable in some aspects. What makes it interesting is the character development and how they react to scenarios.

Characters: Meguru is our protagonist, with a noble means for the money. However we no little of our other characters and why they are there. This is what makes the manga a great read. We learn more about who these characters are, why they're playing the game the way they are, whether or not they care about the winnings to begin with. Even Muguru who seems endlessly optimistic at first finds himself falling into a rugged Hell.
This is what makes the manga really interesting. We don't know who these people are but as we learn, people who we thought were simple or good quickly show a dark undertones to them or motivations that aren't exactly pure hearted. It's simple in that sense, exploring the need for survival and darker human motivations, but it does it all too well. Sometimes the characters can't help themselves. Other times their dark side is of their own free will.
I adore all these characters, be they good guy or bad guy, as all of them are written quite well.

The art: The art in this could get quite cartoony at times and very over dramatic. It only did it at times during high stress character moments. Not much else to say. Think Panty and Stocking, only slightly heavier anime tones instead of western

The bad: This series really shoves it's morals down your throat. I understand it being as short as it is, it needs to get it in somewhere and somehow, but it never does it quietly or subtly. It always decided 'OH, here's a good moments to comment on life and humans.' It never really flowed naturally and really hurt how much I enjoyed the ending of the manga in particular.

Life is Money overall: If you want a survival manga, I more recommend Battle Royale or Liar Game. However, if you want a manga that focuses on characters and you can really enjoy unraveling the layers of a person, then this a manga for you.