Need to find "some good guess prices" for flights in early 2013. What would be a good site to use? How realistic would it be to use 2012? Adjustments?

Trying to compare some cruises. Have a Regent cruise (includes air) Want to compare to 4 others that do not include air. I can handle the adjustments for tours, tips, booze, etc. but the cost of flights is more difficult. When you ask an agent to get all this data for you, seems unfair. My agent has better prices than helpfuliness. I am willing to do the work and get the analysis down to two choices first.

Regent is pricy and seems to offer a much higher experience than the 7 other lines we have cruised with in the past.

Pairs I am looking at include SAN to Manaus, return from Ft Lauderdale;, SAN to Buenous Airies, return from Manaus; SAN to Rio, return from Ft Lauderdale; SAN to Rio, return from Miami.