Cheaper than a ground up design. But I think you can start the clock until someone uses a grinder and cuts the gun right off.

From a product development viewpoint, the gun is an uninteresting part. It's better to use something that already has a mature production line and has been thoroughly field tested. It's the vision and control systems they are interested in developing, the gun is just the chosen end effector for this application.

Even when they're ready to start deploying systems like this, there's a lot of value in using compenents that the military already has a lot of spare parts for and that personnel know how to maintain. I wouldn't expect a custom gun until units like this are commonplace.