Probably some nuissence fee in there for the small job. Is he also pulling a permit?
I'm assuming you're using black iron pipe. If you have no unions, he will have to move the stove to undo the flex line from the stove, which then typically requires replacement per mfg instructions. He'll then have to undo each one of the connections back to the place where he wants to insert the tee. Then put those pieces back. He'll have to build the pipe from the tee to the outside, drill holes, maybe crawl under the deck? Add a valve and more flex line to the grill. The gas will need to be shut off and leak tested and inspected and then all pilots relit.
I can see it being a few hours at least.
Is a drip leg required for the grill?