One breast is saggy and the other is perky. What do I need done and how much would it cost?

Hi. Thank you for your question. It is very hard to give any recommendations without pictures. Usually a breast lift is needed to correct saggy breasts. I would recommend you to consult with a board certified plastic and reconstructive surgeon. All the best
Dr Marco Carmona MD 

Hello and thank you for your question.  The
best advice you can receive is from an in-person consultation.  The solution for this will depend on your goals for the size of your breasts.  In patients with asymmetric breasts, sometimes a breast lift and/or implants can be beneficial.  Adding pictures would help in giving you more accurate advice.   This decision should
be based on a detailed discussion with equal input from both you
and your surgeon.  Cost varies based on geographic region and surgeon expertise.   Make sure you specifically look
at before and after pictures of real patients who have had this surgery
performed by your surgeon and evaluate their results.   The most
important aspect is to find a surgeon you are comfortable with. I recommend
that you seek consultation with a qualified board-certified plastic surgeon who
can evaluate you in person.
Best wishes and good luck.
Richard G. Reish, M.D.
Harvard-trained plastic surgeon

Hello dear!!
Thanks for the question and provided information as well. 
The average cost varies a lot depending on the city, the clinic and the plastic surgeon. Unfortunately, without pictures, it is very difficult to give you a proper answer. l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)

Thank for your question
regarding the appearance of the breasts.  Ultimately, the only way to
answer this question is through an in-person exam with a board-certified
plastic surgeon.  Some people require a breast lift.  Some people
require breast augmentation and/or a combination of the two depending on the
preop appearance that you present to your surgeon with; therefore, be sure to
discuss with your board-certified plastic surgeon.  Hope this information
is helpful to you.  Best of luck!

Without the opportunity of at least seeing photographs of your breasts from the front and side, and better yet a personal examination, it's impossible to answer your questions with any reasonable accuracy.
Asymmetry of the breast can be treated in a number of ways according to the individual problem(s). You may need implants with a unilateral mastopexy. You may need implants with bilateral mastopexies, raising one more than the other. You may just need asymmetrical mastopexies with no implants. Or maybe you need fat grafts to make you more symmetrical. 
Possibly you need a reduction on one side. As you can see, the procedure is available are numerous and must be individualized.
I suggest you see a plastic surgeon certified by the American Board of Plastic Surgery and preferably a member of the American Society of Aesthetic Plastic Surgery for a consultation.
Thank you, and best of luck to you.

If one is saggy, then you might need a lift.  It is hard to say without photos. The fee will vary from surgeon to surgeon.

Thank you for your question.  Breast asymmetry is extremely common.  If this is something that really bothers you, I recommend consulting with a board certified plastic surgeon for a full evaluation and discussion regarding your options.  I recommended not having surgery until your breasts are done developing.  This surgery will likely cause between $5000 and $8000 depending on the surgeon you choose.   You may not require any surgery on the smaller, more perky breast.  The more droopy breast may require a lift with or without reduction of some volume.