Ectoplasmic armor's main benefit is the +10 Armor Bonus to AC. To have a permanent +10 Armor Bonus costs (10 ^ 2 ) * 1000 = 100,000gp. The fact it retains it's armor bonus vs incoporeal creatures is rather minor IMO, and you still get the drawbacks of Leather for the purpose of encumberance and armor check, i wouldn't put the total above 105,000gp.
I can't find anything that gives an limited time armor bonus, so i wouldn't know how to handle that exactly, and i'm assuming that 105k is a little pricey for what you would want it for.