How much does it cost to deal with there are carpenter ants in my tree trunk?

The trunk inspection is inexpensive; the money goes into finding out what is inside. Resistance drilling or tomography at the decay column is an added diagnostic charge and is worth paying before funding a removal you may not need. If the tree stays, the recurring cost is light reduction pruning to lower leverage on the weak section, repeated every few years. If it goes, expect a premium over a normal removal of the same size, because a hollow stem cannot be safely climbed or conventionally notched — the crew works from a bucket or crane and takes it in smaller controlled pieces.