$300 for a safe gas line seems pretty cheap to me. I won't work on a gas line for love nor money, not because I don't think I could, but why take the risk, and anyway, in my area it's code to use a liscenced gas fitter. In fact, the box stores pulled the yellow flex gas pipes from their shelves a few years ago. You should ask the installer to explain the reasons for the cost and I'm sure he will be more than happy to tell you. Perhaps he is installing a quick release connection and an inline valve if you don't have one already. I think, also, that the 'shutoff' has to be immediately visible and easily accessed.
There's my 2 cents for what its worth..."ounce of prevention" thing