Tax deductability is a function of Real Estate tax+Underlying Mortgage Interest as a percentage of the total maintenace in the coop. Many factors can impact the Percentage:
The Coop can own commerial or office space that they rent out.
Type of underlying mortage (IE Interest Only)
Temp Tax Abatements
Even the existance of an underlying mortgage.
Higher numbers are usually better.
Low ones can be due to Land Leases (not Good) and No Underlying Mortgage (Good) or even high expenses. An educated agent should be able to tell you what factors are in play, and your attorney will review the financials before closing.
I have seen numbers in the teens and in the 80-90% range.