It has come to me that if your product or service has any real value then you would not have to knock yourself out on promoting it. A site that has info will naturally be linked to by users. All you have to do it do a little promotion to get the word out and let the rest take care of itself. My advice is that if you are not getting a very high ROI then you should quit that product and find something more creative
There is more competition in some industries and often businesses find it necessary to "knock themselves out" on promoting in order to stay afloat in those industries. There's money to be made there but it's difficult, but the product or service can still be of real value.