I like free. And that's what T-Mobile is charging for their base iPad data plan for new customers, according to Apple's website. You get 200MB per month. What happens if you go over that limit? It's not yet known, but they'll be holding a press conference today to give more details. You can check out their network coverage here to see if this might be a good option for you. Apple's page also shows the rates for the other three major carriers. AT&T starts at $14.99 for 250MB. Sprint's price is also $14.99, but you get 1GB of data per month. That's certainly the best deal if you need more than 200MB. Verizon's base plan had been $30 for 2GB, but they're now offering 1GB for $20.
How much data do you need? I have Verizon's 1GB plan on my iPad mini, and although I use it every day for things like browsing the Internet, checking email, and looking up the weather forecast, I've never come close to exceeding my 1GB. If you use FaceTime or Skype via cellular data, or if you listen to streaming music or watch videos via your cellular data plan, then you can expect to use up your allotment at a faster rate.