That's not counting beyond boosters, value boosters (only sold in major retailers) or Secret Lair drops (a massive headache, as Jeff noted), ancillary products like playmats and tokens, tickets to MagicCon, digital cosmetics like companions, or Avatars in Magic Arena
Yes, rarity matters, but a card's financial value is driven by two things: collectability and playability
They simply buy all of the $50 copies of Greed until there are none left for sale and then immediately list one of the copies for sale at $300, effectively (at least, in the perception of a lot of players) increasing the price of the card x6
These creatures will disappear at the end step, but not before the player gets to push some damage across the table with them
The Teaching App lets you try the game using a Starter Deck
Its a Pauper staple for some sideboards, and its value is entirely based on last being printed in 2006