If you are going to use an ad network (adsense, etc.) you will make almost nothing. Way, way, way less than the $1,200 you are currently bringing in.
Here is what I would recommend: Offer your potential clients a free trial. Let's say 30-90 days. If your product is valuable to them, you will be able to convert them to paying customers. (Like your neighborhood crack dealer. Give it away for free, get them addicted and see who comes back for more! Its a proven strategy.) The key is to get them using your product.
Also, there is no rule stating that you cannot show display ads/text ads AND charge for your product. The only people who complain about ads are hipster/early adopters. If your customers are not redditor types, you can most likely have your cake and eat it, too.