Run, don't walk to your nearest online ticketing site (I recommend AOLTickets.com, but I'm biased)...
Madonna has released new seats at every one of her Madison Square Garden shows, at all price points. My first search yielded $360ish tickets in row M!!! I decided to go for the $169 tickets in the middle. All that goes by way of saying that you can DEFINITELY snag good seats right now. But act before the word gets out, or you'll be back to considering shelling out way more for tickets from resellers or other fans.
Here's a quick link for New York:
Here's a quick link for her full tour: