I agree with James. This would be key in up sale conversions. I know that the proration  is calculated by the second and would always be inaccurate, but even an approximate cost would be good.

I will presume you are using the API for product migration.  Below is .NET code that always seems to be accurate, we display exactly what James suggested when people Upgrade (Downgrades take affect at next billing cycle). Additional code calculates relevant taxes and coupon discounts, but it means I have to track a lot of stuff my side to make it accurate.

An API method to get this value would be much preferred. 

        public decimal GetRemainingValueOfSubscription(string externalSubscriptionID, ISubscription chargifySubscription)
        {
            try
            {
                if (chargifySubscription == null)
                    chargifySubscription = _Chargify.LoadSubscription(Int32.Parse(externalSubscriptionID));
            }
            catch (ChargifyException ex)
            {
                ThrowSubscriptionProcessingExceptionFromChargifyException(ex);
            }
 
            DateTime convertedCurrentTime = DateTime.UtcNow;
 
            TimeSpan subscriptionTimeLeft = chargifySubscription.CurrentPeriodEndsAt.ToUniversalTime().Subtract(convertedCurrentTime);
            TimeSpan totalSubscriptionTime = chargifySubscription.CurrentPeriodEndsAt.ToUniversalTime().Subtract(chargifySubscription.CurrentPeriodStartedAt.ToUniversalTime());
 
            decimal costPerMinute = chargifySubscription.Product.Price / Convert.ToDecimal(totalSubscriptionTime.TotalMinutes);
            decimal subscriptionValue = Convert.ToDecimal(subscriptionTimeLeft.TotalMinutes) * costPerMinute;
 
            // Adjust the value if there was a slight error in the calculation due to time differences
            // and apply the minimum charge
            decimal minimumCharge = new Decimal(0.01);
            if (subscriptionValue > Convert.ToDecimal(chargifySubscription.Product.Price) - minimumCharge)
            {
                subscriptionValue = Convert.ToDecimal(chargifySubscription.Product.Price) - minimumCharge;
            }
 
            return subscriptionValue;
        }

So I'm reading through Docs and notice there is now migration preview support in the API.

I have no idea when this was added but I think it was very very recently and there is no mention of it in release notes.

Cmon Chargify, please monitor these forums and respond when relevant updates are done.

http://docs.chargify.com/api-migrations