Heater on full is about 5 kW usage rate (power).
Run for 15 minutes preheating (1/4 hour) => 1.25 kWh of energy used from the battery. Perhaps you had to use 1.5 kWh to charge the battery that much.
So, call it one and a half kWh to make it easier to do the "higher" math.
Then, whatever you pay for one kWh while charging, say 16¢, ...
(here comes the higher math) ...
times 1.5 => 16 + 8 => 24¢
OK, maybe not HIGHER math, but later 3rd grade math, HIGHER than 2nd grade!