I just paid a not-so-small sum to upgrade all three of us to United's Economy Plus. My husband needs the extra legroom. We are on a total of four flights, so it adds up. We didn't have to do the upgrade on all four, but his comfort is important. We'll cut back on the expenses elsewhere.
Back to the original question, the cost of airfare is certainly impacting our decision on where and when we cruise. We actually cancelled a British Isles cruise for July 2008 (that's not a typo) as I couldn't find airfare to London that was less than $1,000 per person. (We've factored that into the budget for our cruise for next year.) For the amount we had budgeted for our original trip, we were able to rebook for Alaska in a very nice cabin and still have plenty left over.
But for us, it isn't just about the $ when it comes to flying down early/staying a day late. Other than the summer, we have little or no flexibility in our schedules as my husband is a teacher. This means that our cruising will likely be during the summer, unless we are able to find something out of New York or Boston in the spring.