I too am looking into bench signs. I spoke with one company that wanted $100 a month for a bench ad. After that I decided to do it myself. I got a $20 permit from my city and will spend another $175 on a bench and another $100 on a sign.
Then after dong that, I found a company that will place a bench sign for only $20 a month plus a $40 one time set up fee. So, I would shop around.
One suggestion I would have is try and place the bench on a busy corner where there isn't a bus stop. That way your message will get out instead on being blocked by someone's body.