What to do if I have a registered certified tradename being used without my permission?

You can send the infringer (the one infringing on, or violating, your trademark) what is known as a "cease and desist" letter, demanding that they stop doing so. The letter should provide proof of the fact that the trademark is registered. You can also in your letter offer to allow them to license the trademark from you. If they won't stop using it and won't pay you what you consider a fair licensing fee, you can then sue them, seeking both monetary damages and an order barring them from violating your trademark.

There are no minimums or maximums: the amount of potential damages, if you  sue and win, will be based on the circumstances. The larger, better established, and more lucrative your business, the more valuable your trademark, and the more you could potentially recover. For example, Domino's Pizza can recover much more in damages for trademark infringement than our local, one location "Mr. Dino's Pizzeria," even though Mr. Dino's has it all them in terms of quality, at least in my opinion.

In terms of what you could negotiate as a license fee--you can ask for whatever you want. They'll pay as little as they can, bearing in mind the value of using the mark. Presumably, if you're both motivated and flexible, you'll meet in the middle--basically, you can get whatever  you can get them to pay.