What would the additional cost of a tummy tuck be if a panniculectomy was done covered by insurance?

Depends. The additional costs involved are the surgeon's fee, the facility fee, and the anesthesiologist's fee. Insurance usually pays a tiny fraction of the full cost of a cosmetic surgery.
Panniculectomy. Insurance reimbursement for a panniculectomy is very little and i agree with my colleagues that 'paying additional for a tummy tuck' is ethically questionable and can be considered insurance fraud. Most surgeons know better and do not even give this option to you.
It won't . Insurance will not cover an abdominoplasty... You're not really supposed to "pay the difference" to"upgrade" to an abdominoplasty. I have heard of cases where the insurance has denied the claim and the patient ended up with a heft bill.
Be careful. Insurers are getting wary of this and some might deny coverage of your operation afterward and you will get a bill for the whole thing. I do not do these for this reason.
Panniculx=skin only. A panniculectomy is a skin removal procedure. However a tummy tuck also includes other interventions to achieve enhanced body contouring such as rectus muscle diastasis repair, suction assisted lipectomy (liposuction), thigh lifting, mons pubic reduction/lift, umbilicoplasty and many other procedures. Furthermore it may not be advisable to perform these simultaneously insurers may cover some/none.
Varies. The cost for a tummy tuck will vary according to the geographic area. Discuss this with your surgeon. A typical tummy tuck is $7, 500 in my practice in the el paso area. You can expect about a 30-50% discount if your panniculectomy has been approved.