As explained in the other FAQs here, each airline has a base price for making changes. If there is a Birthright class seat available on the date you requested, then that will be the cost.
However, if there is no Birthright class seat available on that date, then you may be offered a higher class fare. If your friend received a lower price, please check if their flight is on the same date, flight, and route (destination airport) as yours. If it is the exact same flight and still lower cost than yours, then likely they requested the extension before you did, or perhaps theirs was processed at a time when there was an available seat.
At any rate, Gil Travel NEVER marks up a higher fare seat. If the price is higher, that is what the airline had available at the time of processing your request and we passed it on to you.