I thought like you at first but I believe he is right. Prices are not really fair despite they are adjusted based on League level. A player who's leve 3,4 or even 10,12, etc must also invest some money in the buildings, while those after level 20 do not need and for them buildings investments ain't cheap at all. Second at high levels you can earn much more money then on low levels, that's why top teams have dozens of billions, plus of course they do not need to buy so often especially if they remain to the same level.
Good point honestly!