We will pay the debt of the 40 year

We will pay the debt of Marmaray for 40 years: Part of the Marmaray project was opened on 29 October. Many comments have been made about its degradation and water intake since its opening. But the main question was the size of its cost.
According to the news on Vagus.tv, the cost of the Marmaray tube tunnel project was $ 5 billion. The project, which has a submarine length of 1.4 km, was borrowed to Japan International Cooperation Bank, Council of Europe Development Bank and European Investment Bank with a 40-year maturity.
So, if we compare Marmaray with other tube tunnel projects in the world, is this figure really high? Here are two examples;
1. Seikan Tunnel in Japan
The part of the tunnel, whose total length is 53.85 km, passes under the sea, is 23.3 kilometers. The cost of this tunnel is $ 3.6 billion. Considering that the part of Marmaray that passes under the sea is only 1.4 km, it will be seen how much this cost is.
2. UK-France Channel Tunnel
The total length of the tunnel, which has a total length of 50.45 km, is 37.9 km. The cost of this tunnel is $ 10 billion. The tunnel connecting the two countries and the submarine part is 37.9 km, approximately 24 times of the submarine part of Marmaray.