(idiomatic) There ain't no such thing as a free lunch; something advertised as being "free" will invariably have hidden costs.
Example: 1966, Robert Heinlein, The Moon Is A Harsh Mistress,
"Gospodin," he said presently, "you used an odd word earlier--odd to me, I mean..."
"Oh, 'tanstaafl.' Means 'There ain't no such thing as a free lunch.' And isn't," I added, pointing to a FREE LUNCH sign across room, "or these drinks would cost half as much. Was reminding her that anything free costs twice as much in long run or turns out worthless."
1985, Lance Parkin, The Infinity Doctors, page 229,
'Tanstaafl. There ain't no such thing as a free lunch. We both know that I was brought here for a reason. If we could get down to business.'