If you're already operating a service business--a lawn service business in your case--you're already 80% of the way there.
For information on cleaning and janitorial businesses, including niche businesses, go to http://www.bscai.org.
For marketing, you'll really use many of the same marketing techniques you're already doing. They should be directed to the listing agents, not to the banks. And, again, http://www.bscai.org
has some good resources.
The numbers that are sometimes quoted--the "up to $2,500 per property"--are generally quite optimistic. A real estate agent with an REO listing (on which he or she probably will receive a reduced commission for more than the average amount of work) isn't likely to spend a whole lot of money on a property trash out.
For more information specifically on starting a trash-out business, see http://bit.ly/Cleanout1