Join Date: Feb 2011
Bikes: Specialized SL2 Roubaix Comp
I think that what you want to do is to just swap out the rear tire for the trainer, not buy a whole wheel with a cassette. You might want to look on craigslist or ebay for a used wheel but most people will want to sell them as a set. It looks like the 2.1 comes with a Shimano 105-5600 or 105-5700 11-28 cassette. It's probably a 5600 if its a 2010 model.
The Shimano 105 cassette is about $90-100 by itself and a cheap wheelset will cost you another $350.
Somebody I know who trains a lot for triathalon told me not to buy the special trainer tires but to just go out to Canadian Tire and get a cheap 700c x 23 tire. He found them to be harder than the trainer specific tires and that they last a lot longer (not to mention cheap .. less than $20 as opposed to $70).