Tough question as there are many variables. These include local labor rates which vary widely across the country, how hard to access the valve and therefore how long the job will take, whether there is a trip charge on top of the actual labor performed, and whether the gas connection has to be brought up to current code or not. Since you've already had the valve replaced, I'm guessing the water heater was no longer under any warranty and that you decided to not replace the entire unit.
Based on lots of plumbing work I've had done in several States, it would not be unreasonable to expect an appropriately-licensed plumber to charge a $75 trip fee, plus hourly rates about $75/hr - $150/hr or more depending on distance to your location, local demand, local supply of qualified plumbers, etc. Call a few plumbers locally and ask them how they charge for what you want. Better yet, see if anyone you know recommends a plumber and contact that person first. The other variable is how long it takes to actually change the valve. The plumber will have the knowledge to do this quickly, but your configuration may take longer than expected due to things like:
1- can the valve be accessed easily?
2- is there enough room to get to the valve and use tools efficiently?
3- are there any gas leaks in the gas lines?
4- does the gas line have to be brought up to current codes?
5- are the valve fittings a straight replacement or does the gas plumbing have to change?
Any of these will require more time and therefore be more expensive.
Source: Replacing a lot of water heaters.