I've never yet earned money from screenwriting. That's what my day job is for.
Surprisingly, there are standard rates of pay for screenwriters, courtesy of the Writers' Guild of Great Britain. I didn't even know there was a Writers' Guild of Great Britain.
It seems I can expect to receive at least £2700 for writing the treatment, at least £9450 for a first draft script, and at least £3240 for a second (or fourteenth) draft.
(For interest, compare the Writers Guild of America rates - almost double on average.)