One other thought on cost... if going lighter means you can increase your speed from 12 to 15 mph (as in your example) that theoretically means you could do a 1500 mile tour in the same time it would take you to do a 1200 mile tour. What cool things might you see in those extra 300 miles? What's the cost of missing those extra 300 miles with no increase in effort?

I know that's only theoretical and probably wouldn't work out exactly that way in the real world, but if cutting your gear means you can ride easier, faster, smoother, etc., it's worth considering.