It depends on a number of factors, but I'd say a group of eight will probably be in the $20-$25 per hour range. Again, there are factors to consider. The English centers around Hanoi typically pay teachers in the $15-$25 range. You know that they have to charge an aggregate rate considerably higher than that to cover overhead and have a viable business. You should be able to offer them a reduced rate than they would likely face. The amount of reduction should depend on your ability to teach what they are specifically looking for, the resources you can supply (i.e. materials, teaching space, multi-media aids, etc.), and other factors such as travel for those involved (are you teaching at their place of business or are they going to your house?). You should also be making more than you would teaching in another school because you'll have to deal with the notoriously irregular schedule of private students (or you can make them pay in advance like schools so they don't cancel as often), you'll have to organize and update the materials all yourself, and you won't necessarily have the advantage of teaching blocks of classes. Travel between groups of private students can seriously cut into the total class time you can manage during peak hours.

As you can guess, there's a lot of room for negotiation here. In the right situation, where you have a highly developed and tested curriculum set up for a group of demanding professionals, charging $30+ an hour would still be a significant discount on the rates they'd get in big centers. If you're just throwing together bunch of mixed level acquaintances with only vague goals, $15-$18 would keep it all on a very casual level.

Good luck.

Thank you so much alpha for taking the time to reply in such detail!

I have a better idea now of the market prices.

Good luck!

=)