Notice that the armors with minimum enhancement bonuses have a cost of "special"?
It means that you can't buy non-magical versions of that armor. Magic armor of +4 or better is always made out of the "masterwork" versions of the base armor, and the price of the "masterwork" is included in the cost of the magic armor. (PHB Page 212, 2nd column)
So leather armor can be enchanted up to +3 (for a total bonus of +5).
But if you want magical leather based armor that is +4 or +5, then it's made out of Feyleather (for a total bonus of +7 or +8).
If you want +6 leather-based armor, then it is always made out Starleather (for a total bonus of +10).
You can't get an armor bonus of +6 or +9 using leather-based armor (barring certain feats), because there are no suits of +4 leather armor, and no suits of +6 Feyleather armor. The Laws of the 4e Universe preclude them.
The minimum cost of Feyleather armor is 45,000 gp (for +4 magic armor [Leather]).