From past experience, I for one would be all for a single up front payment to the agent...
My original contract through an agent for my current client was for 6 months. The agent creamed off his 10%. The contract has lasted over 6 years.
I believe that the Agent would have factored in his profits for the 6 months. Any additional time was a 'bonus' for him....!!!
After 3 years I eventually managed to negotiate a buy out and work for the company directly. The cost of this was a payment from me to the agent of 6 month's commission.
Depending on what the up front 'fee' is it is obviously more cost effective for the contractor if the contract is extended.
Another form of commission I have come accross is a reducing commission over time..... e.g. first year 10% 2nd year 5% 3rd and subsequent years 2%.... This I believe is more equitable, because the Agent does incur costs which deminish over time.
I look forward to seeing the final results of you survey....