@Hydra
Depends on what kind of product you're trying to boost and depends on how much other products that are up there and before those pages are selling. If you search for shoes, it will depend on the sales of the same type of product. On the "all" category though, the same deal, but with every kind of product.
I remember that a friend boosted a female shirt to page 15 or something selling just around 20. A male shoe on the 5th page, I have seen selling upwards of 300 and not going up. So it really depends on luck, dedication and having cash to get something up there.