I think the price is fair and competitive. if you can get a continuity agreement that doesn't end in 6-12months that would be better and justifiable since initially your monthly fee represent a larger portion of their return. later as they get ranked for more keywords and traffic, that fee is spread over the acquisition of more clients, bringing their return on investment up. most businesses expect to pay and have a monthly budget for advertising which they pay as long as they continue advertising. compare yourself to their offline options; yellow pages - a large flat fee for the year, newspapers - a large fee monthly/weekly/yearly (which by the way are also calling on your customers daily to sell them some big "internet package" that costs 10x what you are charging).
At the end of the day it really comes down to the value you are offering, what results you are delivering and how much profit your customers will relize by using your service.
any business would gladly pay you if it is profitable for them to do so.
what specifically are you offering these businesses? and what if any guarantees or results can they expect?