This is often cited as the most powerful card ever printed.

Very few cards are strong enough that they form part of a first-turn-kill, with the vast majority of decks, it's not remotely possible to get enough mana to play the kinds of cards that would end the game before your opponent can even do anything. Black Lotus gives you three mana, without having to play a land. If you're trying to build a hyper-broken combination, it's much easier to do if you have a black lotus.

Even outside of hyper-broken combinations, black lotus contributes heavily towards mana ramp, a mechanic that a large number of decks rely on to get going.

MtG has a lot of extremely powerful cards, but most of those are still somewhat situational. Black Lotus is a card you could add to almost any deck and improve that deck. It's so universally broken that it's impossible to build a deck that can't be improved with a black lotus... unless that deck already has four black lotuses.

As for it's value... they printed 500 of them. Total. The cost you'd pay for a black lotus is "highest bid at auction" - they don't really have a list price, because it's determined entirely by "is there one for sale right now?"