How much is this going to cost? Plenty, meaning at least $20 grand to get a good lawyer to do it. Your sister's lawyer has to file her notice of appeal right away, within 14 days of the entry of judgment, including weekends, I think. If the notice is late, she is going to have to move in the trial court for leave to file a late notice of appeal. Then you need to find a federal appellate lawyer who knows how to do a sentencing appeal. If there was a recommendation by the feds for a certain sentence and the judge disregarded it and slammed her anyway, there was probably something in the case that the judge found to be an aggravating circumstance -- or else he was just an a--hole. Doesn't mean you can't win, but post-Booker federal judges have a lot more discretion to sentence as they see fit, without regard to the guidelines or agreements between the parties.
Your best course would be to talk to a few attorneys who handle criminal appeal work before the federal court of appeals in your district. For many reasons, attorneys may have different ideas about what a reasonable fee, or an appropriate fee structure, should be in any particular case. Any appeal, especially in the federal courts, is a major project, but the only way to get a good answer to your question is to make a few appointments and ask. Don't expect an appellate attorney to give you a quote over the phone. The work is not that predictable.
If you truly want to pursue possible post-sentence options, you need to consult with federal appellate attorneys immediately. It is important to understand that there are strict time deadlines to file certain post-sentence motions or the avenue of appeal may be lost. You definitely want to meet with several lawyers who practice in this field so that you have a basis to determine who seems the most trustworthy, reasonable and knowledgable. Additionally, you likely want to ensure that any lawyer you hire has handled post-sentence matters in federal court on prior occasions. Good luck!
The "average" cost is meaningless. The cost in your sister's case will depend on a host of factors, only some of which you described. Any appeal is very technical and is not for a lay person. As my colleagues said, interview a couple of qualified attorneys and hire one asap.
Evaluating any legal question requires a detailed knowledge of the specific facts involved. Since a short question will rarely contain all the relevant facts, the answer here should be considered a general comment for your consideration and not legal advice.
I agree with the other attorneys who say that it will depend on a variety of factors. At the same time, I also agree that it could cost $20,000 or more (or maybe less), depending on the status and stage of the case. Based on my own criminal law appellate experience, there are always a variety of factors to consider. There are also a number of unanswered questions and insufficient details provided in your question. There are issues as to whether there is a viable motion for post-conviction relief is at issue, aside from the question of a direct appeal claim that is viable. I encourage you to immediately contact an attorney in Tucson with substantial criminal appellate experience.