Made such deal last season, too. The guy had a quite high market value and so I let him train with the team until he reached 4* and then sold him. It was a far better deal than a money transfer. Tomorrow morning I will get my youth players for this season. Lets see if they could fit in my team.
About paying for own youths players I have the strategy to check if the better of both fits in my team and if the token price can be justified compared to auction prices (money + expected no. of tokens that I need to pay to aquire such a player). I agree that it is in many cases too high, but as already said there are cases where a good deal can be made.