When you buy advertising space what do you expect to get back, what is a good number.
I mean like this..
If you lets say buy banner space for $500 a month, how much do you expect to get back from that investment. Three times the invested money or less?
I'm new on this advertising thing, i only worked with my own traffic sources before and i need to set up some goals that are not to unrealistic, and count on how much i need to invest in advertising each month.
I know this question variates from website to website, but what do you think is an OK number, like two times the investment, three times the investment or more.

