The sound that a cash register makes when its drawer is closed after money has been put inside. Used in conversation to indicate that the person being spoken about is going to or already has made quite a profit at little expense.
Steve: Yeah, so I was able to sell the FIFA for $50! Cha-ching!
Fentruck: Good deal. How much do you think I could get for one?
Means "money" is to, or can be, made in an instant.
References the sound older type cash registers made when tabulating a final bill.
Like when someone slips and falls on a banana peel in Trump Tower due to the negligence of the Trump Tower clean up crew to remove that discarded banana peel from its entrance way. That first thought in one's mind would be LAWSUIT!! ...and the first words out of their mouth would be "CHA' CHING"