You need to take their salary into account, not just their purchasing price. Your players may have good stats per dollar spend purchasing them, but the biggest cost is their salary, and in that regard you pay a lot of salary per skill point. Salary is determined at the start of the season, and during the season your old guys lose skills, and this perform less for the same salary. That's something you want to avoid.
Of coursse, if you go overboard and spend huge amount of money buying salary efficient players, you lose the advantage of having low salaries. You need to find a good balance, which isn't easy.