Lower population tourneys you can score under 200 and rank. Higher population tourneys you'll typically need to be over 250, sometimes closer to 300. As far as rewards, you should expect between 200-400 at the lower end based on population, and it goes up from there. You also get your 1 credit. Good luck!
Don't think to much
Try to 2hko your opp if possible
Use a deck with less then 25* good options are: Uppers, Montanas, Recue
Read the informations about dt. What can you do to earn bonus points (more than 12 lifes, win with pillz left)
But the most important information i can tell everybody about dt is playing fast.
Play fast -> more fights -> more points (even if you loose you get points, so don't give up a fight or loosing by timeout).
I hope this will help you
look at the tournament history... you best bet is to look at the previous 2 DTs... for the current DT you'll need roughly the same amount. peetelafoote has the right idea.
for top 1/3 of the DT, about 120 points is usually more than enough to get the 50 clintz and 1 credit reward