Custom Inventory/Player System Price?

Hey guys,

I am asking for advice as to how much a custom Inventory system would be.

(The Heart of the server/ main user interface)

Something along the lines it can be made compatible with majority of addons in order to store entities, weapons, ammo etc

Display the players character with a ui to equip weapons and gear.

Menu Tabs for different sections (help area, admin area, player list)

Its just an Idea at the moment but a little estimate would be appreciated for the cost :)

Cheers