Hmmm, adding +2 Cha and +2 Nat armor to armor is easy. In DMG it reads ability bonus is squared x 1,000gp and any naturall armor bonus is squared x 2,000gp.

But as rule of thumb in many games where I ahve been is said that if you add enchaments on already exchisting item whit base cost you simply said double the price of the most expensive item and then add other factors in. Examply would be examply cloak of protection which gives +1 to all saves and +1 to deflection bonus. Normally cloak costs 1,000gp and ring a 2,000gp. Whit the system what I said the price of that cloak is not 3,000gp but 5,000gp (2,000 x 2 = 4,000 + 1,000 = 5,000gp).

So, your chainmail bikini would cost 4,300 (normal cost mithril chainmail) + 4,000 (Cloak of Charisma+2) + 16,000 (2x price of Amulet of Naturall Armor) = 24,300gp. I hope this helps figure out pricing self-made items.