Honestly if you like to arena I would put the 20$ on it simply for MoP perma-tournament server.
If you don't have friends to team with though, I wouldn't bet on it. I'm about 1700-1900 live, but usually capped at 1800 on TR (also never played TR seriously, was a great chance to play other classes I had in full PvP gear to get a feel of it... also makes a great drinking game if your team is LANing together ...)
Kaeja - Stormrage
// Thanks to maybenotquiteasheavy for the Signature & Avatar! //
Usually on TR your rating will be 400 points less then on Live. To get Vanquisher it's pretty easy since most of the teams there are just troll/kids/noobs £$%&ing around. I'd say you would need between 1.6k to 1.8k max ( depending on pop ) to get title. Also I would not reccomend going without partners from Live since most there are trolls/fakers and whatnot. In my opinion it is worth it.
Don't think of $20 for a title. Look at it as a way of improving, playing with different people and enjoying a 'kind of' balanced PvP realm. You can play many different comps and play against people way better than yourself.
You're guaranteed a pet, and entrance to the MoP realm and, fingers crossed, the title. Even if you don't come top-1000, you'll have gained a lot of experience to apply to live where you might be able to push higher and get Rival etc, so you might get a title out of it that way. (I view Rival as better than Vanquisher anyway).
The $20 is more than worth it for the whole thing, not just a shot at a title.