Portraiture is an area in itself. I would recommend first looking at the prices in your area (D.C./Northern VA) by artists of in the same style/level of expertise you are. Call N. Lee Jackson, if she is still in your area. She is a wonderful portrait painter I studied with nearly 20 years ago. She will know your market and is a fine pastel painter. She was living in the Fairfax area.
Next, I recommend viewing the work of others of same style/level of expertise at http://www.http://www.portraitartist.com/
...look at the work of these fine artists and contact some regarding pricing.
In addition, go to National Portrait Society and other conferences to get a line on the industry, see demonstrations by several artists at one time, talk to those in the high-end area to know what to shoot for in the future, and even get a free. live critique from many people in your field.
Generally speaking, pricing depends on the amount of "flesh" that you paint and the amount/level of complexity is in the background. A head/shoulders against a plain Canson paper background would not be the same as against a bookcase or garden backdrop. Nowadays, it also relates to who you studied with, how well you capture the person for the client and the uniqueness of your "style". In your area the clients are more visually sophisticated, and have seen more portraits than in some other locations (Potomac, D.C., Bethesda, Chevy Chase and the like.)
Go to the National Portrait Gallery in D.C. just to study the portraits.
Garner as many affiliations as possible. Enter portrait as well as general shows with your portrait paintings. Continue to do what you are doing in studying, reading, taking workshops with the best portrait painters you can afford, even traveling if necessary. A whole generation of exquisite portraitists is just ready to move off the scene.
Contact a couple of portrait registries. There is one in NYC and one is Portraits South. Again, to get an idea of what clients are seeking, whether to sell framed/unframed, skill level...
And most of all, paint, paint, paint.
Start with your prices low, but affordable. It is harder to lower prices once they have gone up than to incrementally raise them. However, do not sell yourself short. Formulas are fine but are only a starting point. I used to look at and for formulas. They were great at one time, but other things influence pricing...the market, experience, framing, and so on.
If you decide to post a formula on your Web site, it would be helpful to supply two types. One would be cost plus materials, another would be value added. One alone may not give the whole picture.
Sorry to go on and on...these are just a few criterion for pricing. I know it is frustrating not to see numbers. It is kind of like asking "how will I know I'm in love." Coupled with the formulas take in all you can, then start somewhere you feel comfortable. Sell portraits via word of mouth and it will take you where you wish to go, even more so than with other types of art.