Paid "sponsored' threads? Bidding system for highest placement?

[Jah-Hools]

I want to introduce single paid threads. (PayPal) for commercial interests to promote 'specials' and "discount deals". (they are forbidden to out in the forum)

I am not sure what level to pitch the cost per thread and am unsure what to do about the duration of deal thread before deletion (how long the money will buy visibility)
or how to fairly order of deal threads (latest at the top?)

So it occurred to me that I could have some sort of bid system..? (more you pay the more prominent you thread?) (larger text?!?)

Perhaps that's too complex..

Any plug ins or tips & tricks / war stories about running paid threads would be much appreciated.

Many thanks