I try the same drill 4xLongRun on a fast trainer, academy player, old player, 80 average white skill, 100 average white skill, World Class drill, 2-star drill, ALL gain the same 12 XP.

I dont look at xp when training, just the number of skill gain for 1 rest pack.

Remember younger player gain more skill
18-21 100%
22-25 50%
26+ 25%

Fast Trainer Skill Gain
40-59 5.50
60-79 5.25
80-99 4.25
100-119 3.25
120-139 2.00
140-180 1.00

Academy Player Skill Gain (Estimation for 80% price ratio, vary based on price compared to fast trainer price)
40-59 4.50
60-79 4.00
80-99 3.33
100-119 2.00
120-139 1.33

Assuming the player is less than 22yo fast trainer and for eg Fitness is 120 and Speed is 100 (both white skill), for each 15 condition, the player should be gaining 3.25 skill, and then result maybe Fitness 121.6 Speed 101.6.

Assuming the player is less than 22yo fast trainer with DC role and for eg Fitness is 120 and Speed is 100 (average skill quality is 110), for each 15 condition, the player should be gaining 3.25 skill, for each 2 white skill gain, you gain 1 gray skill, and then result maybe Fitness 122 Speed 101.

To get a streamline squad, target only white skill when age is 18-21, so when the player reach his prime age, you may have 200+ in defensive/attacking skills, and gray skill in minus. Having player in 1 Role only is also a bonus (less white skill to train, less green, lower team quality, higher performance)
example for
DC - Warm Up Gym, Press The play
DL/ DR - DefendingCross, LongRun, Carioca
AMC - BallControl, ShootingTechnique, SlalomDribble

18yo 40% with playstyle is the best candidate, with the right training regime, you can use the player for ~15 years (depending on the level you are playing), so for every 1-2 season, only need to concentrate training on 1 player.