(1) Co-ops are less expensive and much more common than condos. They're more restrictive as well.
(2) Uptown is less expensive than downtown as a rule
(3) Near Columbia University is relatively cheap by NYC standards, but still safe. You occasionally see two or three bedroom fixers between $250k-$350k, which is what I'm looking for
(4) HDFC housing (buildings that got subsidized loans from the city and tax breaks) is cheaper than normal, but has income restrictions (could be pretty high like $150-180k) upon initial purchase. Advantage is lower monthly condo charges/taxes. Disadvantage is that you have to sell to someone who meets the restrictions and generally pay a % of your profit upon sale to the building and/or the city. No big if you're planning to stay there for a while. HDFC buildings tend to be more common downtown in the Lower East Side/East Village.
(5) Parts of Inwood are still cheap and even attractive, but faaaaar from everything else.