Cheap fares are out there. Bestfares.com is a good place to pick these up (don't need a membership).

In terms of lowest cost per mile, there are some incredible deals that come up.

Recently had SNA to CMH (orange county to columbus, OH) for $122 roundtrip, no Sat stay. In terms of pure cost, not cheap, but in terms of mile value, a great deal. I know someone who bought enough of these to make platinum status by February, all for less than $1500, getting 100k in miles to boot. I'm sure there have been some even better deals out there.

Perhaps we should open this thread up: not just cheapest ticket, but best mile value. I'm interested in what's been the best.