We're not going to allow an artificial imbalance in the economy.
Then you should be able to easily
compare transfers with other similar players (age, best skills), or even supply an average price. It's difficult, even for someone with some experience, to know the correct price just my looking at the transfer market.
Also, if you want to secure a "fair" transfer market. Why not just give players fixed prices for any given age/stat combination? Teams could decide (or even the player could decide randomly) who he wanted to sign for. Then there would be no1 cheating any1 - rookie or veteran - how you manage your training, arena, etc would decide what players you would be able to attain.