Turkish Airlines Ticket Change and Additional Fee Policy

I needed to change my THY flight ticket and did so at least 5 days in advance. They asked for an additional payment of around ₺763 for the change. I don't understand how charging an extra fee of around ₺763 for a change made at least 5 days in advance is fair or just. I would understand if it were a ticket cancellation or if I were rescheduling a ticket for the next day, but demanding around ₺763 for postponing a ticket 5 days ahead is absurd. When I checked the ticket prices for rescheduling, my original ticket cost ₺3201, and the total for the new date was around ₺2803. Even if the deduction was ₺763.68, I would still need to pay around ₺400. However, they said that the new ticket price does not apply for changes and there is an extra charge. This situation reminded me of the historical character Deli Dumrul. I want THY officials to know that I feel my money was unjustly taken.