Is it because theyre super special, vanishingly rare, almost legendary in their scarcity
Sensei's Divining Top Sensei's Divining Top is another original member of the Modern banned list, but instead of power-level concerns (although it might be too good now, thanks to the miracle mechanic and Counterbalance, giving Modern access to most of the pieces of the now-banned Legacy Miracles deck), the main reason the artifact was banned is because it slows the game down to a halt
Remove a +1/+1 counter for the creature to gain flying until the end of the turn
Knowing how much a card is worth is easy now, thanks to the ability to trade and sell online to anyone in the world
Used copies average around $650, but this can rise pretty rapidly even with a bad PSA grade - so providing its still in one piece, it's definitely worth getting the card graded.