I like to give the customers two options. Option #1 is pay per catch and I make it high per animal assuming I catch less than I expect to. As mentioned above, this is a good way to advertise as it shows your confidence in your work. Option # 2, I then give them a set price that is the minimal amount that I will do the job for. This covers me in every area and typically saves the customers money over paying per catch due to how I set it up. They will normally go with the set price due to it being smaller, even though there is no guarantee.
For example, I had a fox job where I knew there were four problem foxes. After figuring out my travel expenses, I told the customer that it would be $105 per fox (105 x 4= $420) or the set price of $370 no matter how many I caught. The first option showed him my confidence (showing that I would take the job even with no guarantee of getting paid unless I was successful) while the second offer gave him a firm price that could save him money. He took the firm price (as usual) and I caught all four problem foxes, made him happy with his choice, and I made money.
This is working good for me, I'm not going to change. Hourly seems like it could potentially scare people. Customers are not just paying us for our time, they are paying us for our equipment, knowledge, experience, and time. Hourly wage makes it look/sound just like they are paying us for our time.
I am open minded and would love to hear what is working out there and will try something new if it might be better.
This is a good post keep it going.