I generally look for domains that make $10 a month or more. I make money after about a year but in the mean time, I'm constantly getting backlinks so that I get more than just type in traffic.
Focus on domains that are more affordable. Go for domains that make AT LEAST enough to cover its registration fee (standard registration) per month. This will help you build a solid portfolio. Generally, you can get these domains for less than 3 years revenue, and usually no more than 2 if you look hard enough. My domain, apple-pie.com, pulls $1 a day minimum and $4 on a good day. I paid $135 for it. At its current rate, at the very least I've doubled my money this year.
Too soon to cash in.