A few things here fellas...
Whoever said show money is not taxed is ABSOLUTELY INCORRECT. Show money is a form of income and unless that artist has residency in one of the few states that do not have income tax, they are DEFINITELY TAXED! The issue is that unlike most of us who get their taxes taken out before they ever see their check, show money is left to the artist to report to the IRS as income so that proper taxes can be assessed. This is the problem that many rappers that have tax issues down the road (waddup Young Buck) run into. They get a check for 10k for a show, and spend 10k. 3-5k of that 10k belongs to the man.
Second, that's the value of metals TODAY. By the time an artist gets a piece of jewelry, it has already been bought and held, likely for months at that point. So it's not worth (to the seller) what gold is worth TODAY. Also, without getting too much into distribution channels, jewelers buy jewelry from certain channels that buy precious metals in bulk. A lot of times, jewelry that is sold to these rappers were other pieces of jewelry that was melted down and reformed. Basically, there's a lot of recycling. ALSO, the range of precious metals is HUGE. There's a BIG difference, both in quality and price, between an top notch diamon and a flawed diamond. Same goes for gold, silver, and platium. (Before anyone says some monkey ****, yes, I know diamond is a mineral, not a metal...get off it). 20k gold is MUCH more expensive than 10k gold...
Finally, a lot of people have already touched on this, but a lot of times, jewelry is rented and or borrowed for promotional reasons.
In a nutshell, how do rappers afford chains?? 1) they dont 2) they can afford crappy quality 3) the IRS is essentially "loaning" it to them...they'll come around and collect soon enough. Even my man Nas got hit up by unc sam...