Since you are paying for them, can be risky, especially if you get caught and penalized. The thing is, how would Google know it’s paid? Unless they endorse it with something like Sponsored Link, or something similar I doubt they can actually prove you spent cash on it.
From the “is it worth it” point of view, I am guessing, you have to check the increase in the earnings(if any), and compare them to what u spend monthly on that link, or links.
Bottom line, I am for it.
Out of curiosity do big sites sell links, someone something like CNN 
?