So on the srd Item cost
Command word item Spell level x caster level x 1,800 gp Cape of the Mountebank
This Cape of Montebank cost 10800
That would be 4x9x1800.
What am I missing on this cost?
Not all magic items follow the formulas. Some were just given a price that sounded right.
The formula should be spell level x caster level x 1800gp. So 4x9x1800, or 64,800gp. However, its only useable 1/day, so the cost is divided by 5, for a final cost of 12,960gp. WotC must of though that was too high, so set it at 10,800. Paizo just copied that for Pathfinder. (3.0 D&D did have the cost of the cape at 12,960gp, so it was changed for 3.5 D&D.)
Thanks Jeraa
You missed the next part of the math description.
Since it's useable x times per day you divide the price by (5/charges). The Cape can only be used once per day, so the price is divided by 5.
That puts the price of a custom item with a similar power level to the cape of mountebank at 12960 GP.
Furthermore, if you reduce the CL to 7, the minimum to cast a 4th level spell, then it would be 4*7*1800/(5/1) = 10080 GP.
Finally, Jeraa is right - not all magical items follow the formulas exactly, which is why the rules also encourage you to look at similar magical items to avoid having the rules broken in two.
For instance you could make a ring of continuous Infernal Healing for pocket change (spell level 1 * caster level 1 x 2000 GP for continuous), whereas the ring of regeneration is priced at a hefty 90 000 GP. Both items perform a similar function and should be priced at roughly the same level.
Edit: Rargh! The alternate rogue kit strikes again.
thanks Kudaku for the insight missed that 1/5 mechanic