Well, I have to be honest, my family is in Venice, so I always fly into Venice, and I have never researched what other cities could be cheaper to fly to.
I guess it depends on the season and demand for each city, sometimes local festivals or events increase drastically the number of tourists, and the price of the ticket goes up. It requires some planning ahead, and some research, probably a travel agent with the right software can answer this very quickly, but I don't have those resources.
One thing is sure, the Italian public transportation, including the railroad service, connects every corner of the territory, so it would be possible to land in any region and then get to your final destination by railroad or bus. But then you'd have to take into consideration that cost and the extra time traveling.