Need help with a custom wonderous item.

[Laharal]

Hello fellow playgrounders,

I came across a quite interesting guide from Guigarci
for factotums. In the item section, something caught my attention

Fancy Hat: A combo of my three favorite head items: hat of disguise, circlet of persuasion, headband of intelligence. Starting off at 9850gp for the 2 related abilities (disguise and persuasion). You get a +13 to disguise, +3 to all other charisma checks (iaijutsu focus anyone), +X to Intelligence


However, I'm quite novice regarding custom items.

1) How does he get to that cost?
2) Do you have to ask for DM's for custom items that follow the core rules?
3) It is legit that an item has 3 powers like that? If yes is there an extra fee per extra power?
4) Should such a tool be considered super-rare/relic?
5) Anything else I should know about this?

Many thanks for you help!

[Troacctid]

To combine two items in one slot, start with the price of the most expensive item, multiply the price of all the other items by 1.5, and take the sum. Common magical effects (per Magic Item Compendium) can be added with a flat cost with no multiplier to an item in the appropriate body slot. So it's 4500 (Circlet of Persuasion) + 1800*1.5 (Hat of Disguise) + 4000 (+2 enhancement bonus to an ability score, a common effect) = 11,200 gp total.
For custom items, yes. For adding MIC-approved common effects to magic items, no, assuming MIC is allowed in your game.
Yes and yes. (See the first answer.)
No.
Chapter 6 of the Magic Item Compendium can tell you basically everything you need to know about magic items, so if you want to learn more, check it out.