Just pulled the trigger on a Specialized Allez (61 frame) road bike, since I am getting back into riding. While the Allez does have a carbon-fiber fork and aluminum frame, I think the wheels could be lighter and question other parts that could be replaced. So I looked at upgrades and noticed a Trek Emonda (~$3k) was not bad. Then looked at some Domanes, and there was an "endurance" for (~$4k) and a Domane triathlon for (~$6k). The endurance Domane was very light and had wider tires, but the triathlon Domane had skinny wheels that were carbon-fiber. Unfortunately, for my taste I thought that it would be better to have either a hybrid with the triathlon wheels on the endurance bike, or a hybrid with the endurance handle bars, cables, brake handles, on the triathlon bike.
Given the above, it's known that you have to pay more for lighter parts, hence, the price-per-pound goes up. Thinking about a plot of price vs. pound for a specific part, a part that gets expensive quickly with decreasing weight will have a lower slope (price/pound) than a part that sheds a lot of weight but costs less per pound --> greater slope.
So as a rule-of-thumb, which parts typically have a greater slope which allow you ditch more weight with less cost? What would be the order of lowest slope per part for wheels, brakes, crank, derailleurs, neck, seat post, cables?
Slope is actually negative for a plot of price per pound. Using the attached cartoon, a strong negative slope implies that for each additional pound, you pay maybe $2800 less, hence slope is -$2800 / per increasing pound -- or, for each decreasing pound you pay $2800 more. Ideal part would be as each pound is added, you pay $300 less and not $2800 less, same as, each pound lost costs $300 more - Utopia!
(slope means change in price divided by change in weight between 2 or more bikes. The price of a single bike, like $15000 that weighs 15 pounds does not give you slope. Slope requires comparison via differentials with one or more bikes that costs $1000 and weighs 20 pounds).