All of my domains that I bought to test this were basically set up with merchants who had products that related to each keyword phrase. For me, the product's average order value is about $60-$80, of which I get 10%. Each domain only needed to get 1 sale per year to pay for itself. After that, the sales are gravy. Bonus was that one of the merchants had a script that displays the products matching any keyword. Since these domains were all keywords, it just works out beautifully.
These domains were so niche that if someone typed one into a browser, they were very confident of what they were looking for. Conversion was really high since I offered exactly what they were looking for.
I have them all sitting on the same IP. I wasn't concerned with search engines, just the type in traffic. I even have them interlinked since they are closely related to each other. But I do believe I am getting some SE traffic.
My domains are even really only relevent about 3 months out of the year and even the best one gets only about 1000 searches a month in the peak months.
Don't have anything you can sell? Collect leads and find someone to sell them to.