How many of X to scrap to buy new X

So, a thought occurred to me the other day. I have quite a few electric weed eaters procured from CurbCo. I want a new 40 volt cordless to replace my trusty gas/oil that's becoming not so trusty.

I wondered, how many of those weed eaters do I have to scrap/sell to afford the one I want.

Want a nice laundry set, how many washers/dryers do I have to scrap in order to by a new set.

These are not actual questions, just thinking in terms of number of items scrapped to get new item.



Just a random thought while breaking stuff down. Have a good one!
UMR