Depends on a lot of things... How many calves? Broke to tie, or ready to show? Just a quick, rough clip job or will they expect you to work hair and clip them tight? I'd say with that many variables, the easiest way to charge would be by the hour, as JTM suggests. I'd take the $10 per hour as a starting place, but be sure you make it worth your while. Also be sure to get all your expenses.