Suppose it depends on the niche, if it's a niche where each customer makes them a lot of money (e.g solicitor) then they might go for that.
You could sell it from the angle that this is an early bird offer and if they wait until the site is at number 1 the price will be double. Also could say that the early bird offer has a minimum 6 or 12 month contract (However long you think you can get away with basically) in order to retain them. Obviously always offer the full years ad placement for a reduced price, 2 months free for example. At $200 a month per business you could end up with $6000 up front and all you have to do is keep working to get the site to number 1.
Not sure if your UK or not but if you are use a standing order and set it to rebill them every year also, make sure this is mentioned in the contract.
Out of interest, what position are you in at the moment to be receiving the 30-40 uniques a month from the overall 260 searches per month?
