How much does a capsulorrhaphy cost on average?

 I am sorry to hear about your concerns after breast augmentation surgery. Although some “falling to these sides” of breast implants is quite normal when you lie down, if this occurs “excessively”, it may be a concern to patients. This phenomenon is called lateral displacement of the breast implant; it may be of concern from the visual and the discomfort standpoints. Surgical correction is an option.
Generally, the lateral breast implant displacement can be corrected using an internal suture technique, decreasing the size of the pockets and moving the implants toward the midline. In my practice, I use a 2 layered suture technique to close off the space laterally (towards the axilla). The use of acellular dermal matrix is an option ( although not usually necessary) especially if significant implant rippling/palpability is present. Also, the use of a larger/wider breast implant may help improve/decrease the space between the breast implants. When it comes to cost 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… are some of the potential variables involved.
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 revisionary surgery may be more costly in many ways. 
I hope this, and the attached link (dedicated to revisionary breast surgery concerns), helps. Best wishes.

Capsulorraphy is one of the most difficult maneouvres in revision surgery of the breast, do consider the skills of the surgeon and not the price.If necessary do travel to affordable countries where you can find skilled surgeons.

 It sounds like you have diagnosed the problem correctly. The pocket around the prosthesis has stretched out allowing the implant to fall towards your armpit when you lay down. This is a fairly common problem we see and is corrected by either a capsulorrhaphy to reduce the size of the pockets, a larger implant to appropriately fill the pocket, or sometimes a capsular a fee with and ADM insert as well.