How much copper by weight is in one 50ft 20 amp extension cord?

depends on the wire gauge and strand type

sold strand 12 AWG wire is 50.57 feet per pound.

most extension cords however are not solid strand, they are multistrand line which allows for flexibility and breakage.

standard size is 28 AWG, 2067 feet per pound.

7 strand 12-3 line gives you 1050 feet of 28 AWG

or ~$2 worth of copper if its clean and bare, less if it still in its insulator or if the insulator is burned off.