Can I take a tax break for subidizing someone ...

Homeowner's association dues are not deductible for a private residence.  They are deductible if the unit is rented out for income and would be deducted on Schedule E.  You may want to check with the association to see if there is a payment in lieu of the dues for the low income units.  There are several ways this might be structured - the mgt company may be receiving grant money to cover the expenses, or may have received lower cost financing for the project, etc.  Don't assume you are paying more to cover their expenses.  I would think the non-disclosure could be a problem, however and you might have a basis for suit based on that.

Russ, I agree with "Fairy's" comments and would add that the amount you paid is the amount deductible, regardless of whether or not part of that is due to picking up someone else's share. I also own rental property in an HOA and due to the number of foreclosures, each assessment is higher than it would be without the 10% which is uncollectable. I feel your pain.