Breast Asymmetry correction?? How much does it cost for a single implant? (photo)

In your case, I don't think just an implant would give symmetry.  I would suggest a breast lift on both sides with an implant on the smaller side or you could do a fat transfer on the smaller side.

I am sorry to hear about the physical and (possible) psychosocial problems your breast asymmetry has caused. Your concerns are certainly understandable. 
There will likely be several options available to you.  One option would likely involve breast reduction/lifting of the larger breast plus/minus breast augmentation on one or both sides.

My best suggestion: take your time and do your due diligence and select your plastic surgeon carefully. Make sure that he/she has significant/demonstrable experience helping patients achieve the types of outcomes you will be pleased with. Then, communicate your goals carefully as well. Working together, you will come up with the best plan to achieve an outcome that you will be pleased with.  

When it comes to cost of surgery, there may be many “variables” involved. The specific operation planned, the 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, type of breast implants… are some of the potential variables involved. You will find average costs of surgery listed on this website, and on individual plastic surgeon's websites as well.

Best wishes.