How many Skill points = 1 Cone

How many Skill Points for 1 FULL Star

For the first time i bought my Youth Player , a 4 star player , no good yet, but building up , are these Youth players better than buying a 5 star player age 18/21 with a 4 or 9 on the end.

If these youth players are going to be worth buying , i need to grow 2 stars because i will lose 1 on promotion .

How many rests is it going to take and is it worth the effort and money.