The commercial guys here pay $15-20/cow/month for each bull. That will most likely increase this year. About the same as to AI to a less expensive bull. A promoted clubby type bull will run 3500 a month or more. The renter is liable while the bull is on their property. Care and feed can be an issue. If the bull needs vet treatment, the renter should be liable for the vet cost just as if they owned the bull. If the bull wrecks a gate, that is their cost. I once loaned a bull at no cost and the guy told me he did me a favor for not charging me for the gate the bull wrecked when penned next to cows in heat. He also hauled it to the sale barn with out saying anything. As Leanbeef suggests, know the outfit and make sure an agreement is in writing and up front.