I just received an estimate to sanitary weld a 3/8" full union in a 6.5 gallon stock pot that took my breath away. I knew that the job was not going to be cheap, but $250.00 to cut a hole and weld a union is beyond ridiculous.
First you want a coupler welded in not a union (might just be semantics). A normal shop rate is $60-$100 per hour. To cut the hole, build a quick back purge system and then weld probably would be about an hour.
If you're welding on a 6 gallon kettle I'm assuming it's like .6-.8mm thick which is nearly impossible to weld a coupler onto without blowing through or warping the crap out of it. In this case you may be better off with a weldless fitting....
The pot is a Vollrath Optio 27-quart stock pot (1.5mm thick). While not super thick, it is definitely as thick as my first brew kettle, which was a converted Vollrath 38-quart stock pot (what is known as the Classic series today). I had a 3/8" NPT coupler welded into it for fifty dollars. However, that was twenty years ago. and the welder who did the work has since retired.
I would purchase a commercially-made stock pot-based kettle. However, no one makes a nice 6 or 6.5 gallon kettle, and I want to build a 3-gallon brew house with an appropriately-sized kettle. Thirty-two-plus quart kettles are not optimal for sub-5-gallon batches.