Source, in full
In response to a woman in a café in Spain willing to pay for a napkin Picasso drew on. Picasso is said to have responded "Sure, that will be $10.000", to which the woman responded in shock "But it only took you ten minutes to draw that.", after which Picasso responded the above.
Notes from Wikiquote
How expensive is 10 minutes of your time? – mentioned in two books, one from 1984, and another one from 2016, but not verifiable.