The all-action pass master left the Emirates Stadium for the Catalan giants in 2012.
He was a star at Arsenal, but went on to be a bit part player on a high wage at Barca.
"When Barcelona offered me a contract, and I saw how much I would earn, I didn't think twice," he admitted on social media.
"I felt my wife and children should have comfortable lives once my career is over. I met Barca's sporting director, and he told me I would not get to play many games, but I didn't give a f*** I knew that now I would become a millionaire."
Song continued: "Most footballers live beyond their means.
"I was at Arsenal for eight years but only began to earn a good living in the last four.
“That was because my salary went up a lot -- but also because I came to realize what a waster I was.
“During my entire time at Arsenal, I couldn't even save £100,000, while people thought I must be a millionaire."