I'm not sure if adults need that type of incentive. I would think that a melave malka and a prestigious speaker coming to pump them up for holding up the world and their dedication would do as much, if not more, to encourage attendance. Self actualization is a greater motivator than prizes, and is also better PR to boost attendance when guys walk around on a high from the program. It might be better if they think to themselves they're coming purely lishmah. A positive environment can be self perpetuating more than material incentivization.
I see I'm late to this party, but may I chime in with a comment from Rabbi Berkovits?
"The Yetzer Hora is such a powerful force - it would be a shame not to harness it."
When I ran a chazara kolel for the lomdei hadaf, I called the financial incentive "the doron to the Satan" (it was Adar, in my defense). Fact is, we all want to learn. We really do. Just our Yetzer comes up with excuses or pumps up our desire for the havlei hazeman until we give in. You wave a little gashmius at it (like the carrot leading the horse) and it subsides, letting us do what we really want to do.
Moreover, as has been mentioned, when we need to get out of the house to learn, our better halves are more motivated for a regular thing versus the spur of the moment "I really should learn something", which she may think is an excuse to get out of doing the dishes (and she is probably right, too).