When you save a household to library-- which you do from either CAS, or Manage Worlds, but NOT a regular game save-- that household will have whatever its current balance was, when imported to a new game.
For example: I make a new sim, Bob Smith. As soon as I finish customizing him in CAS and click the check mark, the game saves him to the library, and I then get taken to the map to choose a place to live, with 20,000 simoleans to spend. I can also go into other saved games, and I'll be able to drop Bob Smith in any other game, also with $20k.
But, say I cheat Bob Smith 1,000,000 simoleans. On a console, I've just cursed that particular saved game with whatever cheater flag it gives. And Bob Smith still starts other games with the original $20k. So now, I go into manage worlds, choose his household, and Save to Library. It would probably be a good idea to change the name of the household too, because now you have TWO Bob Smith households, and from the library, you can't tell which is which. So let's say the new name for his household is Rich. The Rich household, which contains just one guy named Bob Smith, will start new games or join other games with $1M. You can even share the Rich household to the Gallery, and other people who download him will also start with $1M.
I've got a sim I downloaded, who starts with something like 2,100 simoleans! I'm on a PC, so I don't care about entering cheats, so I might cheat her some money, or I might just roll with it and scratch out a living, starting on a cheap $1500 vacant lot and just $600 to start. Whoever created her probably just saved household to library after playing for a little while, without realizing how broke he just made her.