Believe me or not but in case of you have a account with Digital Point Forum with more than 10k posts than you can sell your signature space for more than 300$ per month. I am not joking.. Some people are ready to pay this much without giving 2nd thought to the fact that the back link which they will get is temporary.
So please beware of this spam. If you really want a quality back link than hire a SEO specialist and give him this much money and he will build more than 10 permanent back links for you.