I strongly disagree with a 95% original rust free 72 510 being worth $3k. Actually if there is one for that price, let me at it!
I would argue that a 2 door 72 510 original with no rust in good shape would easily fetch $6-7k depending.
Decent cars go for $3k.. I sold my 72 510 with Rebello L20, twin Mikuni 44's, Troy Ermish coilovers, Roadtser 5 speed, in great shape a year ago for $12k and the guy didn't even try to negotiate.
The nicest original 510 I know of is Miss Dottie and was for sale a year or so ago at $11k which I feel was a bargain. 35k original miles, original paint etc. I almost traded mine for it at one time 2 years ago.
510's seem to be extremely highly collectible in great shape. More so that S30's IMO as there are far less nice 510's left in the US than Z's.
I have been actively looking at the values and prices of 510's for the last 10 years.
Some photos of my baby. I plan on owning another one day.