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.
He's right. Figure what the dub job is preventing you from otherwise doing. If it's something that can be done as a side process where you push the record buttons and walk away, changing tapes and etc. on your way to and from bathroom and cookie breaks, while doing other profitable work, then you can charge a lower rate.
What if you have multi-channel audio and need to ride a mix? Or weird changing video levels that need proc-amp tweaking just to see well enough to log it? Now you are chained to the rack. That's man-hours, need to be billed at least at half your hourly day rate.
Are you adding a little something for wear and tear on youre transports, or to go towards the gear replacement fund, and a little markup on the tape stock to cover your hassle in obtaining and carrying/stocking it?