It’ll be hard for any of us to give a dollar amount depending on your market, your setup, your typical rates.
I can suggest to you what you should be considering though when developing a rate.
Do you have a separate DVD recorder or is it in your NLE computer?
Do you have to do the dubs yourself?
Is this part of a larger project in which the client is also paying for post production time?
Reason for the above questions.
If doing the dubs takes down your NLE because you’re a small shop then you have to make sure you don’t take a lose by not getting editing done. This can be offset though IF the client is booking (not just THINKING about booking) a major post production job. If you have landed the post job yet, I’d mark the dubs higher. If you have an assistant who is doing this in another room, you can probably mark lower.
If you’re a small shop don’t try to match the price of a big dub house. That’s what outsourcing is for. If the client can’t wait for the turnaround on an outsourced job, you should consider that and keep the price high since they’re paying for your personal attention to the job.
Just some random thoughts. Others may think differently though.