A total waste of time and money doing monthly, regardless of whacky claims by some virtually no directory including yahoo or dmoz won't give you enough airtime to justify short periods of advertising. A year at the least may give you some chance of genuine ROI which is what its all about.
How about this. $3.00 / 1 Month OR $18.00 / 6 Months OR $36.00 / 12 Months So the user can choose which payment they would like.