I don't know the legalities of processing birds for someone else in Wisconsin, but that is something you may want to look into. Likely won't be an issue if the friend isn't selling the processed meat, but better to be safe than sorry.
To answer your question... the cost of processing depends on a lot of different factors. Here in Central Missouri, I pay $2.60 per bird to a Mennonite-owned/operated USDA-inspected facility that processes my birds and packages them for retail sale. There are additional charges if I want the birds cut up or other parts (liver, gizzard, etc) kept and packaged. I assume they have a minimum charge, but I'm usually processing 100+ birds at a time so I'm not sure what it is. As a customer, I am more than happy with this price for the service we receive.
Having said that, I would not personally process birds for $2.60/bird. While I can do it myself and have the basic equipment necessary, I don't enjoy processing and would rather not do it. I'm honestly not sure what price would make it "worth it" to me to process birds for someone else. On some level, that is a personal determination. If you do decide to process for this friend, consider the value of your time, impact on your equipment, and costs associated with utility usage (electric, water, etc) involved.