We are constantly emailed/phoned by affiliate programs looking for premium banner placement. Just as you have mentioned, most of them are only offering a 5-10% commission boost, and usually for just a short amount of time. Hardly an attractive option IMO.
Since we would rather keep the banners running that we know convert, we end up ignoring most of the requests. We do however accept payment for banner placement under one of two conditions:
1) Affiliate Banner Placement – Pay a set amount up front and we guarantee 2 months of exposure. Our affiliate code is tagged to the banner so we can track progress. If the banner converts well, we keep it up, if not, we drop it after 2-3 months.
2) Straight Banner Placement – Pay a monthly fee and we will keep the banner running.
So far the majority of interest is in the Affiliate Banner Placement.