It could be using the wrong amount to calculate your total price (the amount left vs the amount used)
Anyway, create a file called test.php in the same directory as index.php, and paste this into it:
<?php
include 'core/codon.config.php';
echo FuelData::GetFuelPrice('YMMV');
And run the file. Replace the YMMV with your airport (the departure in this case)