It is very simple!
Price will change with Quality, age and training speed.
a 18 years old is more expensive that a 19 years old with the same quality.
2 players with same age and same quality, the one with the highest price will train faster that the other.
Age will change the speed of training.

To gaine a star your player need to gaine 75skill points. Every season you go up a level, your players will lose a star, but there quality will be the same.

Just shearch for forum and you will find lot of tutorials.