These guys are giving you good advice. You asked about the worst that can happen. I'll outline a couple of likely scenarios.
First, consider that registration doesn't happen instantly -- it's takes a good year or two or even more. The $300-600 you mentioned is probably for just filing the application, not going through the entire process.
If you're lucky, the Trademark Office will find the TV show's use of the mark and cite it against you and you get stopped there. That's probably one of your better scenarios.
Suppose you go a little further: your application gets published for opposition. If you're almost as lucky as before, the TV show files an opposition in response to the publication. If you're wise, you'll drop it there. Otherwise, you can have your own mini-ligitation in the Trademark Office and your chances of proving you were first to use the trademark over the TV show are pretty slim.
So, let's say you get even past publication without opposition and the Trademark Office is about to register your trademark. Okay, now all you have to do is submit your sworn statement, under penalty of perjury, that you've used your trademark in interstate commerce and submit one or more specimens, examples of your mark as you've actually used it in interstate commerce. Remember, the sham sales across a state border of the past is no longer accepted. You have to use the trademark in the ordinary stream of commerce.
Even if you get this far (and I doubt very much that you would), in court, you're going to have to show (i) that you're the senior user (same issue you would have had to show in opposition) and that (ii) the mark wasn't registered in bad faith. (Trademark guys, help me out on that -- I'm not sure about the senior user thing; maybe the defendant has the burden of proof but it seems fairly easy to prove in this hypothetical).
If you lose on the bad faith issue (and you're at substantial risk for losing there), there's a good chance you'll have to pay whatever the court thinks is reasonable attorney fees to the TV show. If the court thinks you did a really bad thing (and there's a substantial chance of that), you may be required to pay punitive damages at whatever level the court thinks will teach you a lesson.
I really don't see much of a chance that you could come out of this experience unharmed. I think you'd be better off taking your $300-600 and buying lottery tickets.