"Mill" is a colloquial term that originates from the card Millstone, used here to mean "putting the top cards of a player's library into his or her graveyard." For example, you could say, "In the finals of the Pro Tour Qualifier, Herbert cast Tome Scour to mill away my final five cards, dashing my chances of playing on the Pro Tour." There are many players who will play with Tome Scour in their decks
Casting spells for free from your opponents graveyards is one thing, but then having them lose life equal to the mana value when you do cast them is pretty nutty
Nonetheless, we still got something good out of this clunky mechanic
For me, I would probably look at the current sales prices across a mix of websites and comparison shop to get the most accurate value of my Magic: The Gathering cards
Packing a passive exile effect for impulse draw and reliable mana advantage through Treasure, Prosper is both an all-time great Commander and a value engine when slotted into other decks
