So it's not really a mashup. I'm not using the Google Maps API, but the new My Maps functionality within Google Maps. Very easy to use, even with cut-and-paste data.
The data comes from a BusinessWeek article on The Biggest Metro Areas with the Lowest Rents. Kinda neat, gives a nice at-a-glance representation of the cheaper bigger areas to live in the United States. Minus one outlier (Salt Lake City), it's all in the Midwest and South; and 15 out of the 20 are inland. Only two (Houston and Jacksonville FL) are on the ocean.
I've drawn a polygon encompassing the "central" area of large metro areas with lowest rents, and it's interesting that the polygon is centered on the Mississippi River and Ohio Valley. You've also got Texas + Oklahoma + northern Alabama and Georgia, but half of these metro areas are on or near the big river valleys and the Great Lakes.