I'd guess anywhere between $50 and $150. Depends on many different factors that have already been mentioned in this thread though such as traffic source, how many unique visitors (not just impressions), niche, etc...
1) Uniques per month - 15,000 2) Traffic source - google search engine 3) Niches vary, they are a conglomeration of niches, from apparel to poems - assume "general" niche. I am sure that someone below me will post "It depends on factor A, B, C" because it seems like the standard way to increase post count, and if it is said below then I will link to this post.