In general, if there is ever an item that everyone would want, and would be silly not to buy at the price given, it's not allowed in my game. So, you have to compare.
For this, you do indeed need to find something else that gives these effects and compare the price.
Here is how I would do it. First, make the most expensive effect the one that's not slotless. That saves money…
Robe of +4 Armor AC = 16,000
Then add…
+2 Deflection AC slotless = 16,000
+2 Natural Armor AC slotless = 16,000
+2 Luck AC slotless = 20,000
+2 Sacred AC slotless = 20,000
Only useable by monks = no effect, as the PC in question is a monk.
So that comes to 88,000 Gp. That's the neighborhood it should be in. If you are making a home-brew item, and it's much cheaper and better that what's in the DMG, you should adjust it.
As for the "only monk" thing, I can see how that would make a shop pay less for it, but I don't see how that would make a monk pay less for it. I live in Japan, but am a native English speaker. When I see English books here, I don't care if I pay extra. But, the native Japanese people wouldn't pay much for them, so if I sell off my old books, the shops give me less.
Also, in my game, I wouldn't allow a robe of armor +4 for 16,000. To me, an armor bonus doesn’t match with a robe, so there's a 50% surcharge on it, for a price of 24,000.
Long story short, I wouldn't allow this in my game for less than 96,000 or so retail, something like 34,000 wholesale if it had the monk restriction on it. (I know those aren't game terms, but in my brain it's easier for me to think of shops buying wholesale and selling retail).
Have fun!
-Tatsu