learnin' money
The money that you lose in learning a valuable lesson which you had to pay for or had a hidden cost.
"My first poker night with the boys cost me $400 in learnin' money."
Or
"I didn't read the fine print and had to pay my learnin' money when I broke my contract."
Or
"I didn't read the fine print and had to pay my learnin' money when I broke my contract."