The song "Hey, Big Spender" from the musical 'Sweet Charity,' has previously been parodied on both of Matt Groening's other animated series. In 'The Simpsons' episode "Two Bad Neighbors" Homer sings a version of the song at a yard sale; and in 'Futurama' ("Leaving Mars Vegas") the song is "Hey, Big Lobster" and plays when Dr. Zoidberg arrives at the casino with lots of money.
The title "Hey Pig Spender" is a play on the big, dramatic song "Hey Big Spender," which was very popular in the 60's & 70's, and has been used in many movies and TV shows since its debut in "Sweet Charity"--especially anything mob or spy related. It's a play on words here as Merkimer is still a pig, but they're trying to fool his parents into giving them money for their defense effort.