What is the average cost of a reverse abdominoplasty to address the loose skin below my breasts and above my belly button?

It depends on your examination and how much skin needs to be excised but you are looking anywhere from $3000 to $5,000 in surgeon's fees.  Please see a board certified PS who can examine you and go over your options.

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. You will find some average costs of surgery listed on this website and on individual plastic surgeons' websites (including attached link) 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.  Best wishes.