What Wicked neglected to state is that a Rotary is a PITA to actually install into the car. There is nothing easy about making it bolt up. From the folks that I've raced with that have actually swapped one in, they've all stated that they would not do it again. Unless there is something that I am not privey to, I'd avoid a rotory at all costs.
Anyway, depending on what piston engine you are willing to install into the Miata, 1.6's can be had for roughly $200 on ebay and less elsewhere. Transmissions go about the same. Depending on who you talk to, you may even get a killer deal (I.E. I scored a free Miata driveline, engine included, just by B.S. with some random guy visiting the local junkyard).