Well there's the Otome, Mini Mosu, Tatane, Lihoi Chun, Elixir, Candy Jack, Melluzine and Cell Kamikaze deck. That's pretty cheap. Unless you want to win the fights and not the whole Deathmatch?
With a deck like Thoazol's I've gotten over 300 points in a DM once. Doesn't always win, but is definitely the easiest and cheapest way to get lots of points, at the loss of the meaningless win/loss ratio. If you're worried about price there's not much you can do, Extended and even e25 are packed with decks that are expensive and strong.
Let's make this simple and easy:
• Anything that has high 2HKO potential and a decent amount of power passes as a Deathmatch deck.
•Best mode to start off in would be Standard Dm if you're talking about how much a good deck would cost.
Find your own playstyle to play around. Other people's suggestions are not always for you. and anyways, you become a better player off experience, everyone makes mistakes and research what their playstyle is. And once they achieve this, they truly become someone to fear when you see them in DM.
Standard DM is easier and you can do alright with a cheapish deck. Try Vortex clan or Sakrohm / Uppers and use Hugo or Morphun as leader . And what Asian said^.... (I remember seeing him in DM quite a few times and I knew I was gonna get my butt kicked lol)