Okay, here we go:

On vayama.com, search for flights from IAH to TBS, leaving on 11/15/2011 and returning on 12/7/2011. Plug in any number of adult passengers you want; max is 6. Choose the first flight option in the search results, and the total fare should be $-7,434.70. It seems some numbers have been scrambled in vayama.com's calculations for the fare; in any case, when I tried to purchase a version of this that actually wasn't negative (child tickets are subject to "normal" pricing), that's when I got the "undefined" error.

Give it a shot; see what you get.