Steve I put together a package on TireRack for about $1200 excluding shipping. Not too bad for a dedicated winter setup.
4x 18x8" Sport Edition A7 black painted wheels
4x 225/50/18 Michelin X-Ice Xi2 tires
This of course assumes you don't have the sport package brakes.
If you are going to be driving on snow/ice this should be a pretty good setup. Obviously you are going to give up a lot of handling and grip in the dry, but with this setup and some weight in the hatch over the rear wheels you 'ought to be able to negotiate snow-packed and icy roads without too much hassle.
Edit: I forgot to mention, if you do buy from TireRack make sure you go through the TireRack link at the top of the Wheels & Tires section of the forum here:
http://www.the370z.com/wheels-tires/ It helps to support the forum.