How to research past purchase price of sponsor apt?

If your patient read every page. At the very least you should be able to find the amount of tax paid on the transaction. Then reverse calculate.
I'll leave it an expert to post what that tax amount's percentage is supposed to be for I don't know.

Maybe there are some things that you aren't entitled to know.

...says Pandora, having learned her lesson.

Why not entitled?

One of the nice things about buying a co-op apartment is that details of the sale by and large are kept private.

I am continually astounded at how wrong VP of Coop Board NYCMatt can be.

You would think he would have a 50% chance of getting things right by guessing, yet his "wrong" average is more like 90%.

As of the middle of '05, co-op sales price figures (based on transfer taxes) are available on ACRIS. Very often, the data entry is not 100% accurate, so do a search with the following parameters:

1) Name of buyer
2) Name of seller (for sponsor sales, look up previous sponsor sales for name of corp.)

There are many misspellings in ACRIS, so do a search by
3) Block & lot # (there could be multiple lots #s for a particular building)

If this fails, please send me the address of the building (nyc10023a@yahoo.com) and I'll find it for you, gratis.