Where does it end?

Are you asking the buyers, the vendors, or both here?

It doesn't for most people ! Fun Hobby and depending on your taste in equipment not cheap.

“Last month I spent $4,300 on a pair of *used* speaker cables.”

Well, you didn’t say what brand of cables or details of rest of your system but $4300 is lot of money for used speaker cables. May be rest of your system is say over $100K to justify the four figures speaker cables :-)