It largely depends on the way in which you are promoting, but for most people VOD sales mushroom and Pay Site Sales are somewhat static.

The longer you promote VOD the more sales.

I'm always biased since I run Hotmovies, but my traffic generally converts around 1 in 30 for VOD, 1 in 200 for pay sites (unless I hit on something hot - sometimes a pay site will convert incredibly well for me for a few weeks)