A really nice, inexpensive solution is the GNU Radio USRP, designed by Matt Ettus. See:
It's really just a radio front-end/back-end: It uses your PC for data processing, which makes it a lot more flexible and cheaper than someone's DSP board. On the downside, there'd be a fair amount of effort to take anything you designed with it and turn it into a self-contained "box," but even this isn't that hard these days... there's a guy out there who has a kit that does this, although I'm not able to lay my hands on it at the moment (it's an HF rig using a small form factor PC running embedded Windoze). Anyway, the point is that for research, it's hard to beat a USRP on a price/performance basis assuming you already have access to a decent PC.