Have you tried giving the callers a percentage of the profit? It might create higher quality leads and more sales.
Yup, there are different methods of how to motivate the cold callers. See I started my own business and had to do cold calling. Sometimes I had days, that I couldn't believe myself - my success rate was like 100 % - e.g. 10 calls, 10 appointments. Other times it was shi**y and I called 20 people and got only 1 appointment. There are many factors that you must consider, but you have to have something valuable to sell. My stuff was selling for $1000+ so I didn't need many appointments to live off of it. You just have to try, try, try and maybe you will have clients like I do now: "I couldn't reject you. There just wasn't any opportunity."

So now I call nobody, because everyone says YES! And it didn't take long... Few years of cold calling and you are where I am now with phone calls.
But back to motivation: Percentage of sale may be a better motivation to cold callers. But only in case your sales people are closing a lot of sales. Otherwise you may also earn a bad name. And also percentage of sales must be higher than if you were paying for leads (I mean in absolute money).
And don't worry about paying someone a lot of money - heck it is even easier if they quadruple your biz...