The installation of a shut-off valve on the appliance gas line should cost no more than the price of the shut-off valve and the service call time to install the valve. You may also have to pay for the small section of flex tubing and fittings to go from the valve to your stove. I charge a flat rate of $60 (US) for the housecall (to a locale in my city) and $100 per hour (pro-rated at $25 per each quarter hour of work done) for this sort of job. I’d guess that for the job you describe, the cost would be $60, plus $50, plust $30 for parts. About $140. Given the vagaries of geography, I’d say that $200 or less would be quite reasonable for this simple job.
You might be able to DIY for $30 and your time, IF you were competent and already had the tools. It sounds like you have your doubts, so hire a plumber. Mistakes with gas can be deadly.
NtP