anyone know why the cc increase drastically after 10 years? goes up to almost $4/sf. seems like a good apt

No they didn't and don't. The co-op has owned the land ever since the building converted in 1983.

The co-op has no mortgage at all. They paid it off in 2006 with part of the $4,100,000 they got from Extell for air rights. (The new http://streeteasy.com/nyc/building/one57-condominium is to the south.)

One57 has huge mortgages in the hundreds of millions, which're cross-recorded under the co-op's block/lot because of the air rights. They're probably what you saw.

This apartment is just a share-heavy high-maintenance penthouse. The new owner will pay $4191 for now, with the usual increases. The seller is kicking back $240,000, either in one whack or spread over ten years. I don't know why the seller doesn't just reduce the sales price and be done with it.

Here's a story about the air-rights sale: http://www.habitatmag.com/Earlier-Issues/2007-November/Featured-Articles-from-Our-Print-Magazine/Selling-Air-Rights

The PH is over-weighted with shares, especially now that there'll a 90-story building looming next over it.

It has 381 shares, while #5B (http://streeteasy.com/nyc/sale/684129-coop-152-west-58th-street-midtown-new-york) has 102. A big premium for a few hundred square feet, an extra bath, and terraces.