“You choose whether your equipment falls to the ground in your space, merges into your new form, or is worn by it. Worn equipment functions as normal, but the DM decides whether it is practical for the new form to wear a piece of equipment, based on the creature’s shape and size“ so up to your dm 

how much would it cost ina magic store

That would be entirely up to your DM as nowhere really has any rules on specific magic item prices, the DMG and XGtE have vague prices by rarity but not all magic items are created equally.

As a DM, I use this for magic item prices:

Sane Magic Item Prices

On there it has them listed at 6000 GP 

At the top of the first post you can download as a PDF as well.

Do the bracers work with mage armor 

Quote from Anti08 >>

Do the bracers work with mage armor 

 Yes, as Mage Armor isn't worn or actually armor.  It's a spell effect that happens to have the word "armor" in it.

Any

Monumentally stupid artwork, great item tho.

you just going to go to the bottom of your inventory and you can attune to it

Did you read the description, only works with no shield armor. 

Ah these are perfect, I'm annoying my DM for I can get the highest AC possible, by being a tortle with killer dex (I know they save dex doesn't affect it, but my dungeon master is fine with it), their also a forge domain cleric, a fighter (defense +1 to AC DM counts natural AC as general AC), and shield. I have a 27 AC now, and one of my spells is shield of faith. 

The question is can a Monk wear these and add to his armor class?

yes

trash

It does recognize it if you click attuned.