material cost
Material cost is the money you pay per kilogram, and it is very often the property that actually decides the design. Brilliant engineering that costs too much simply never ships.
Cost per kilogram spans about six orders of magnitude: concrete around 0.1 US dollars per kg, structural steel around 1, aluminum around 2, titanium around 30, carbon-fiber composite around 40, and precious metals far above that. But what usually matters more is cost per unit of function — cost per unit of stiffness or strength. You can build a cost version of a performance index, for example C_m x rho / E (where C_m is price per kilogram), and rank materials by that just as you rank by a stiffness index.
The honest caveat: raw material price is only part of the story. Processing and manufacturing often dominate the final cost of a part, and prices swing with markets and availability — a scarce metal can jump in price overnight. So 'cheap per kilogram' and 'cheap to build a working part from' are not the same thing.
Concrete costs about 0.1 US dollars per kg, structural steel about 1, aluminum about 2, titanium about 30, and carbon-fiber composite about 40. That is why bridges and buildings are concrete and steel, not titanium — even though titanium is stronger and lighter, its cost per unit of load-carrying capacity is far too high for a structure that needs thousands of tonnes.
Six orders of magnitude in price per kilogram usually decide which material family is even allowed in the room.
Cheap per kilogram is not the same as cheap per part: a costly material that lets you use far less of it, or skip machining, can win — final part cost, not raw price, is what matters.