From what I know (and I'm no expert....)
1) It's a lot more rare than a 5513, which is the Sub you'd get from that era for that price.
2) It's Seiko's first "professional diver" and so has some history.
3) The bordered indices are actually a little nicer than the contemporary sub (later examples did have bordered indices)
4) In 1967 a sub cost $210. The 6215 was similarly-priced
5) the Seiko has an early 1 pc. case rated to 300m. The 5513 had a standard screw-back case rated to 200m
6) the 6215 is a highly tuned 35j movement; the Rolex 1520 movement was no slouch (27j; hand-finished), but many, many more were produced, so again, rarity.
This may be a bit high of a price for this condition, but IMO the 6215 stacks up nicely against a mid-1960s sub. Certainly more interesting to me than a new GS or Spring Drive, but I like vintage....
What makes a beat up Seiko worth the price of a mint Sub?
Serious question, as I'm still learning about Seiko.