Cost depends directly on the hourly rate of the development team. However, it might be reasonable to hire an experienced team so that both business logic and 3D graphics work smoothly. An AR app is another shop window, so it must demonstrate your products in the best way.
Also, you need a license for an AR engine. The annual cost is about $2K.
You also need a server to run back-end calculations and someone to maintain your infrastructure (which is a typical infrastructure when you maintain an e-commerce web or mobile solution).