Here is my formula -
Send a sample post, it might cost you lets say $X.
Measure the income on that post, make sure you setup up your measurement/analysis code before hand. Analytics is a very important part of IM business. I wont repeat that again.
Say your income on the post/traffic received by the post over a week = $Y.
Page Worth = $Y x 10 x Niche Value
Niche Value ranges from 0.1 - 1 , depending on the niche you are.
CPA Freebies Niche , Niche Value = 1
Adsense Niche , Niche Value = 0.2
Other niches can have their own niche values, which can be calculated by mere experiments.
Again, Niche Value is individualistic calculation which I did over several months on my various web properties after measuring ROI on the page posts, I bought.
Hope this helps.