I view the celestial armor as "Divine Mithral" better than the actual material, made that way through magic. And the incredible Dex allowance should have a scaling cost of a [plus], not a GP cost modifier IMHO and the only other armor it belongs on would be a chain shirt.
Celestial Armor: This bright silver or gold +3 chainmail is so fine and light that it can be worn under normal clothing without betraying its presence. It has a maximum Dexterity bonus of +8, an armor check penalty of –2, and an arcane spell failure chance of 15%. It is considered light armor, weighs 20 pounds, and it allows the wearer to use fly on command (as the spell) once per day.
Faint transmutation [good]; CL 5th; Craft Magic Arms and Armor, creator must be good, fly; Price 22,400 gp; Cost 12,550 gp + 1,004 XP.
The cost is between +4 and +5 armor. As far as Upgrading the enhancement on it, I'd cost upgrading it as upgrading +5 armor, adding +11,000 to the base price to make it +4 or adding +24,000 to the base price to make it +5.
+4 armor 16,000 gp <<<Generous costing
+5 armor 25,000 gp <<<Conservative costing
+6 armor/shield 36,000 gp
+7 armor/shield 49,000 gp