I feel that it all started with the 50th anniversary of the 911 last year and then lots of press and public interest in SINGER's 964 creations have really helped to push up the prices on ALL of the nicer 964s. Typical supply and demand scenario.
I feel very, very fortunate that I bought my current C4 cab last year, before the prices really started going nuts.
Don't want to spend $500K on a Singer? Then buy a very nice 964 for $50K, while you still can...
PS: Mine is not for sale.