About how much is a reduction from 36C to a 36A or very small B?

Without photos it is impossible to know what exactly you need.  A typical reduction should be in the range of $10,000 to $12,000 inclusive of all costs.  It sounds like you may only need a lift.  Also be cautious not to get your breasts over reduced.  They often go down in size considerably in the months after surgery.  Good luck!

Dear breastreduction101, the cost of surgery depends on a lot of factors (type of surgery, number of treated areas, anesthesia fee, surgeon, facility fee, city, etc.). It would be the best to visit or call plastic surgery offices and ask for a price.Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

Thank you for the question.  
When it comes to costs of surgery, there may be many “variables” involved. The specific surgeon's credentials/experience level (or lack thereof), the specific geographic area you are looking at, the different operations available, the location of the procedure (quality of surgery facility), type/quality of anesthesia provider, insurance company involvement (if any)… are some of the potential variables involved. You will find some average costs of surgery on this website and on individual plastic surgeons' websites as well.
In regards to outcome of the planned procedure, more important than costs of the procedures will be careful selection of your plastic surgeon. I would suggest starting with the American Society of Plastic Surgery and/or the Aesthetic Society of Plastic Surgery to obtain a list of well experienced board-certified plastic surgeons.
Then, I would suggest you visit a few surgeons whose practices concentrate on aesthetic surgery. ***Ask to SEE lots of examples of their work AND preferably speak/see patients who have had similar procedures done.
Be very careful with your decision-making. You will find, while doing your due diligence, that there are many different “specialties” who will offer their services to you; again, I strongly recommend you concentrate on surgeons certified by the American Board of Plastic Surgery.
  I would also suggest that you do not base your selection of plastic surgeon primarily on costs of surgery; keep in mind that further revisionary surgery may be more costly in many ways.
Because you mentioned specific cups sizes, some thoughts may be helpful to you. Before undergoing the breast reduction procedure it will be very important to communicate your size goals with your surgeon (who you should choose very carefully). Most patients wish to achieve enough of a reduction to help with their symptoms while remaining proportionate with the remainder of their torso. 
With the goal of improving communication with my patients I find the use of photographs of “goal” pictures (and breasts that are too big or too small) and computer imaging are very helpful. For example, I have found that the use of words such as “natural” or “A or small B cup” means different things to different people and therefore prove unhelpful. Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup size may also be inaccurate. 
 Best wishes with your breast reduction procedure; hopefully you will be very pleased with the outcome of the procedure performed.