Spell out numbers in dialogue unless they are excessively awkward.
"You owe me one hundred and fifty-five dollars," he said is preferable to "You owe me $155," he said. Numbers, and also the dollar sign and percent sign, somehow do not look right in dialogue, although we do accept them in quotations in newspaper accounts; newspapers do not follow this rule.
"The materials were $122.36, the labor comes to $88.50 plus $43 for overtime, and payoffs were $1,250 to the city and $10 to the doorman, giving us a grand total of $1,413.86," he said would be very tedious if all the sums were spelled out. They could be spelled out if the writer wants to stretch them out for effect, but they are easier to absorb as figures, and most readers would be much less put off by the figures than they would be by one hundred and twenty-two dollars and thirty-six cents. So figures are okay when spelled-out; numbers are unacceptably awkward.
Exception
The only general exception is years--they are always in figures in dialogue unless the writer wants them said in an unusual way: "It was in nineteen-ought-six," he said, "and long before anyone foresaw the ruckus that came in nineteen and fourteen."
Depending on the requirements of what they are writing, writers can decide to make any other exceptions they choose. "We're expecting the probe to be closest to Venus at exactly 2236 hours" and "Don't use this stimulant if the temperature is below 96.5 or above 101.5"could both written out without excessive awkwardness. However, if military time occurs constantly in the context of the first example and body temperature occurs constantly in that of the second, a writer may justifiably decide to use figures.
