My barn owner doesn't charge for that kind of stuff. She does it just for the love of the horses, and wanting them to be taken care of the best possible. I work for her and we all just help out with sick or injured horses with no charge to the owner. Obviously a vet needs to come out, then the owner has to pay the vet bill, but that's about it. When I brought my horses home for the winter and one coliced and she came the 30min to my house and gave my horse meds and helped me keep him up and walking free of charge. Another girl's horse recently had an abscess, and my barn owner allowed her to use her medical supplies for free to treat it.
She built her barn herself too. Nothing was handed to her, she's the only one in her family who loves horses. Really it's up to you what/if you charge for it. Every barn owner is different and has a different situation.