"We were immature and we got caught up," Pratt tells In Touch. "Every time we'd go out to eat, we'd order $4,000 bottles of wine. Heidi was going to the mall and dropping $20,000 to $30,000 a day. We thought we were Jay Z and Beyonce."
In 2011, the couple came close to filing for bankruptcy after Montag spent $30,000 on plastic surgery the year prior.
Their financial woes will be outlined in the upcoming E! special After Shock: Heidi & Spencer, which premieres Monday, Dec. 9.
Montag tells In Touch she feels "like a different person" with her more frugal spending habits now that the paychecks have stopped coming in. Looking back on The Hills, she tells the magazine that the show "took a toll on my soul. I'm thankful not to have fake fights anymore!"