A poll on LinkedIn asking just that question got 500+ responses. Here are the results (note, the amounts are in British Pounds):
While revelatory in itself, segmentation always yields additional insight. Here we see the cost per business-to-business telemarketing lead broken down by company size:
What I find noteworthy in this second graph is that the smallest businesses are more likely to pay the highest prices for their leads. My suspicion is that this is caused by economies of scale issues.
Telemarketing campaigns require substantive administrative overhead- even for the smallest of projects. The percentage of administrative overhead contributing to the cost per lead increases the smaller the project is. Given that small companies, in most cases, can only afford small projects, it makes sense that they end up paying the most.