The Giants have PSL's for every seat and they are pretty much sold out. They do sell a handful of seats but those are very limited and hard to get. I picked up 2nd row endzone seats back in 2010 from them when I needed 2 extra seats for some friends, and those went out to PSL holder's first.
I went to the Giants vs Saints game this past season, tickets were $95.00 a pop for 2 for a grand total of ~$200+ in the Lexus Mezzanine A Club level and had a blast. The key was that I could get to the Stadium in about 45 minutes from the Port Authority Bus Terminal and I got them off StubHub that day around noon. So your best bet for the cheapest tickets possible is to buy them off of StubHub a few hours or so before kick-off.