Why price of coins is high expensive by gems?

Why price of coins buyed by gems in the shop is very high expensive?!.how  2.5million buyed by 25000gems 

This should be fair in buying for example 50millionby with15000 coins and 2.5millionby with 1000gems 

And rewards in ranked level is low 200 gems

These should be higher level,higher rewards level 

....else