You can do this by searching itasoftware and a 300 mile radius and restricting results by fareclass.
For instance from lax

from lax::x? x? x? x? x? dl+ /f bc=t
to nyc;ord;fll (etc, lots of cities with overlapping 300 mile radius) and select 300 mile radius.

do this for a day or two from now and you'll see what's on sale. You should try U and L as sometimes those are cheap as well.

MKE is a good city as mentioned earlier (at least from LAX).


Once you have the cities with cheap last minute fare, you can do an extended month long search with just those city pairs. You can also setup Expertflyer to notifiy you when a low fareclass seat opens.