What did the chief priests buy with Judas' thirty pieces of silver?

Answer: The potter's field

Since it was blood money, they bought the potter's field as a burial place for foreigners — called the Field of Blood.

“The chief priests picked up the pieces of silver and said, “It is unlawful to put this into the treasury, since it is blood money.” After conferring together, they used the money to buy the potter’s field as a burial place for foreigners. That is why it has been called the Field of Blood to this day.”— Matthew 27:6-8 (BSB)