I priced out a few options recently. I wanted to compare getting new 16" wheels+tires, new 17" wheels+tires, or getting just 18" tires to mount on my OEM wheels.
Here's what I found:
16" steel with Blizzak WS60 mounted: $588
16" steel with Blizzak Revo1 mounted: $496 (Revo1 was clearanced, can't guarantee they still have any)
17" alloy with Blizzak WS60 mounted: $940
18" Blizzak LM-22 tires
alone: $768
I don't care what my gf says, I'll be rockin 16" steelies this winter!

Just have to decide on what tire... I ran Revo1's on my Focus a couple winters ago and they were ok, but my friend's Civic with WS-somethings blew it out of the water. Especially if Revo1's are no longer in stock, I'll probably get WS60's.
edit: this was all from Tirerack
double edit: Oh yeah, I was at my dealer recently and one of their guys was telling me about a MS3 he'd seen come in with 16" wheels. Those too were ordered as a Tirerack package. If you have any doubts, call them up, the sales people are usually extremely knowledgeable and helpful. Tire shopping is always fun, enjoy!