The additional effect of the Bident costs one colorless and one blue mana and requires you to tap it. This tap effect makes it so that all creatures that your opponents control must attack during that turn if able. This can be a fantastic way to really mess up an opponent's strategy. At 4 mana, the Bident is towards the top of the mana curve in Constructed decks. But that didn't stop it from having 2 to 3 copies in a number of highly competitive Mono Blue Devotion decklists.
While Bident of Thassa isn't really playable in Modern or other 60-card Constructed format, it's an extremely good EDH/Commander card. That tap ability can badly screw up your other opponents' attack phases. Also, the card advantage that the Bident provides you is extremely important with 99-card decks. While it's not played as much as some of the other artifacts from Theros, it actually sees more play than the more expensive Bow of Nylea and popular Spear of Heliod.
The price of Bident of Thassa is actually quite low considering its playability in EDH. However, this is because of two reprints in a Commander 2015 pre-constructed deck and in Masters 25. Even foils from Theros and Masters 25 are about $1 as of March 2018, and that's just too low for a card that's this useful in so many decks. If you're looking to pick up this card, grabbing a foil copy from either of its foil printings is probably your best bet for the best long-term value. Bident of Thassa is a great card that can give you value in a couple of ways and is worth considering in a wide variety of decks.