FWIW, I am aware of several AM daytimers that do well in very small markets. The company I work for has a 5-kw AM daytimer on a low clear-channel frequency that has a significant audience (as confirmed by Arbitron's County Coverage) and does a good bit of business in a town of about 10,000 and a county of about 45,000. They do a very good full-service morning show--the source of most of the ad bucks--followed by the Rightwing Unholy Trio--Beck, Rush & Hannity, a tough sell in local direct markets except to attract GOP campaign dollars in a year like this.
But it is part of a cluster with FM sticks that generate 3X the revenue... and the morning crew also functions as VT talent for the FMs... and cost of operation isn't separated from the cluster--so it is hard to tell whether it could stand on its own. Probably not. But we'll never know.
My last thought on this subject is this:
The Top 40 format--and format radio, in general--started on an AM Daytimer: Omaha's 660/KOWH. It started on an AM Daytimer because the owners (Storz Brewing Company) had to do something different because they couldn't compete with fulltimers KFAB, KOIL, WOW and KBON. They had to take a risk.
Within a few years KOIL switched to Top 40 and their 24/7 signal killed KOWH. And they flipped 660 to something else (Easy Listening, I believe, as KMEO). That has always been the story for AM Daytimers. If you can find a viable format that no one else will program in your market you may be able to make some money for awhile.
Just recognize that it will likely be a temporary success, and have your next idea lined up.