You can spend as little as $500 and as much as $2750 on a tablet and tablet data plan (not including taxes, which I left out of this article entirely). That's an insane range, and depends on a ridiculous number of variables.
Look at your smartphones and the amount of data you use for your smartphone. See if you can add a mobile hotspot feature to your smartphone rather than pay for a separate 3G data plan for a tablet.
The bottom line is that you really need to research how much data you really need, and what combination of hardware and data plans really fits your budget.