I don't imagine your costs are less than ours in Alberta.  I won't take anything on for less than a $1.00 per day.  Charge for all the work you do including checking and salting and consider the size of the animals that come in.  There is no doubt that a 1600lb cow with a 5 month old calf eats more than a 1300lb cow with a 3 month old calf.  If they are stockers then charge yardage and per lb of gain.  It would be a great time to hone your rotational grazing skills.  Reading Judy's book is probably the best advise so far.