martinibuster - 11:20 pm on Sep 11, 2011 (gmt 0)
Unless they have control over the links it can be difficult to guarantee a set amount per month. The outcome depends on an unknown third party.
There are several ways to control the links. A common method is to expend effort into lining up an inventory of sites that will reserve a block of links for a guaranteed yearly price. A lower level method is spamming forums across a range of topics to build the inventory.