Air Fare

It's a crap shoot. My opinion would be to grab the $704 (which I agree is very reasonable) and try to get it as a round trip if possible. For an extra $700+, I'd definitely take the train back to Venice from Rome!

But do check an open jaw tickets because it could be cheaper when you add in the ground transportation and time to return to Venice.

Kayak shows open-jaw flights into Venice and out of Rome, random dates in October 2 weeks apart, for under $1200 RT (Swiss, Delta, and Us airways all have options with 1 stop in each direction from SFO).

Hopefully that incl all the taxes and fuel surcharges in the $704.oo. If not you will have sticker shock , I just priced SFO-to Uk rd trip in June w/ $350 in fuel surcharges! Do a comp search at www.kayak.com, or farecompare.com. Oct-Nov the fares are usually better but yes Venice can be more pricey than other european hubs.
I would never buy a one way tix at sep. times when you really want a rd trip.

I priced an open-jaw (multi segment) ticket from SFO to Venice and Rome back to SFO for the last week of October-mid November. I got some Alitalia flights through NY for $1011. This price included all taxes. I used itasoftware.com in the "classic version". I would go directly to the Alitalia website to confirm and book. This doesn't seem like such a bad fare for off peak travel. Its not a fare sale but its not a bad price either.
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I would recommend looking on yapta.com. If you haven't heard about it, check it out. You can tag all your possible flights, and yapta will check the prices several times a day and will e-mail you when prices drop. They check the prices on kayak, orbitz, etc. as well as the airline. I found that prices vary greatly, sometimes day to day. I jumped on a $957 fare from Tampa to Brussels and out from Rome. Turns out, a few weeks later, another airline had them for $800. That was the low, but sometimes fares jumped to over $2,000 (yes, that's correct), but only stayed that way for a day or two. It's fascinating. Also, most airlines will refund the difference if you book through them and the price goes down more than $150. Yapta will track that for you too and will alert you if that happens. Didn't help me, of course, because I was on Continental, and it was US Airways that dropped so low.