A regular prorate would yield 5/12 x $780 = $325. That is provided, of course, that you paid 12-month premium in full five months ago, to the effect that the insurer did not advance you any of the premium; and that the cancellation was not for your breach, such as for missed or delinquent payment.
Only under the condition in which carrier (hence the underwriter) withdrew solely of their own volition would you typically be given a full, prorated refund. That is because the original premium was predicated on actuarial evalution of risk-reward to insurer based on a minimum, annual premium term. Naturally, an actuary would judge risk-reward to be less favorable for, say, one month; or for five months. Additionally, you have right to ask for an explanation of how premium was determined if what you received seems unreasonable.
Only under the condition in which carrier (hence the underwriter) withdrew solely of their own volition would you typically be given a full, prorated refund. That is because the original premium was predicated on actuarial evalution of risk-reward to insurer based on a minimum, annual premium term. Naturally, an actuary would judge risk-reward to be less favorable for, say, one month; or for five months. Additionally, you have right to ask for an explanation of how premium was determined if what you received seems unreasonable.