How much money should you invest to get listings?

I am down to two listings and need to get some new ones.I purchased monthly calendars, door hangers and even had flyers made up to send out to fsbo's. I did however recieve and get a listing from a calendar I had sent out about a year ago.I'm not having the best of luck this year and wondered what is the limit ( money wise) you should spend. It can get real expensive doing mailouts and driving all over town looking for fsbo's. Any suggestions? thanks