For this guide we will use 50T for quality insurance each season for both players starting the season after the power training has finished to keep players the same relative quality when they lose a star each season.
You have also got to consider each season to maintain these players ability you will need to invest around 50 greens on both players to make up for the lost star with promotion each season to keep the same player at the same ability, If it costs 450T each to get these 2 players to about the same rating, then it would cost 800T to insure Miku for 8 seasons and 600T to insure Michu for 4 seasons until they reach age 26 and the development rate slows down for the player, then you would have to spend another 600T raising another 22 year 6* scout for 4 more seasons when Miku has turned 22 and is already still at a high ability and only insured, So it looks like the younger 18 year old players (if they are fast trainers) are 1.5x more effective to power train than your scouts. Thats if of course you have all the tokens in the first place.
We also believe that the fastest training players in the game are your 24T 4* youth players that most players refuse to pay 24T for that the youth academy generates, would be interesting to see how one of those being power trained would compare to these 2 players here.