Is there a fee you can charge for being a power attorney?

What duties are you doing, in general you only sign if they are not available, and can not decide anything against their wishes. More often it is a family member.

Example I gave my son a limited power of attorney to do a real estate probate issue for me in another state.

Any fee would have to be discussed and agreed to by the person who has given you that power, you could not charge anything without their permission,

The short answer is yes. There can be an administrative fee if the power of attorney is in effect and being used. Normally it isn't much if you're a family member.