IDEA does surveys on instructor salaries. Here is a link to an article with some info you might find interesting. Specialty instructors is a broad category, but gives you some idea.
You do not say where you are, but remember that this is going to vary widely geographically. You might start googling other Tai Chi teachers in your area and see what a range of published prices are.
Also remember that the rates will differ if you are teaching in a health club, a stand alone studio, in a corporate setting, or as your own business. But of course your overhead will also differ. You also have to factor in things like insurance.
The other thing that will have an effect is experience and background. A teacher with many years experience, especially within a market where they have built name recognition will be able to set higher rates than someone breaking in. A teacher with training and certifications recognized in their own industry as top notch will often command higher rates. If you are just starting out you may need to set rates at a slightly lower end, and/or offer introductory specials, and/or take some work in established studios to learn the business and make a name.
Pricing is relative to your location and what you offer.
Things to consider:
1. What is the average price of similar programs in your area
2. What kind of experience do those instructors have compared to you
3. What is your overhead/how much do you have/want to make
4. Would you rather play the volume game (cheaper but bigger classes) or quality game (more expensive, but more intimate)
5. What can you offer that the other places can not
Based on your answers to this you will know if you can price higher then the others in your area, the same, or you need to go lower. My true advice is to charge what you think you are worth, offer things no one else can, and make sure you are offering the best product and customer service in the area. This is very much a you pay for what you get industry, do not under sell yourself.