What is important here is that a fair arrangement be made. What is fair is whatever you to agree to; however, it should not favor one of the parties over the other.
In general, yardage costs are variable from locations. Some use a reduced yardage and then mark up feed. Costs are generally broken into thirds, with equipment and utilities one-third; labor one-third; and facility, manure and runoff control making up the remaining one-third.
It appears that you are paying some utilities and also facility costs. I think a fair arrangement is between one-third to one-half normal yardage. Yardage is running 30- to 40-cents per head, per day. So, I think 10- 15-cents per head, per day is probably fair as there is usually 3- to 5-cents profit built in that the two of you should share in this total yardage price.