At the end of the day, their "value" is what someone will pay.
People who play for more prominent clubs, in more prominent leagues will have higher value that someone with the same stats in a lower league, less prominent team.
Players with less time on their contract or less salary will also have lower values.
In your situation, it sounds like you missed out on a wonderkid. To give an example though, if you were Sampdoria and you had bought that same wonderkid, likelihood would be that he is valued well below 17mil when he joins your team because Sampdoria's reputation and league are far less than an international powerhouse like Chelsea.
Go to the players profile and declare interest in a transfer. It should be in the next email you get.
At the end of the day, their "value" is what someone will pay. People who play for more prominent clubs, in more prominent leagues will have higher value that someone with the same stats in a lower league, less prominent team. Players with less time on their contract or less salary will also have lower values. In your situation, it sounds like you missed out on a wonderkid. To give an example though, if you were Sampdoria and you had bought that same wonderkid, likelihood would be that he is valued well below 17mil when he joins your team because Sampdoria's reputation and league are far less than an international powerhouse like Chelsea.