At the top end, graded 10 copies have crossed $22,000, with sales at $23,000 in April and $25,100 in March
The combination of card draw and pressure is quite valuable, because the draw advantage gives you the fuel to end the game before your opponents find a way to get around it or to generate their own pressure
This is the best way to know for sure how rare your card is
Once Into The Inklands arrives, illumineers will be able to summon locations as well
Arahbo's hidden strength is since you're only presenting one threat at a time, you're far less likely to overextend and play out your hand onto the board, meaning creature board wipes like Wrath of God are less powerful against you since you're often trading 1 for 1
It's not quite as rare as any of the others we mentioned early on, but, as Mark Rosewater has said, some such cards could actually be legal in certain formats or events