When you buy tires based only on the price, you usually get what you pay for...
Using the Tire Rack's site, your OEM tires are 205/65-16 (02 Altima 2.5, not one of the upgraded models)
The stock tire is the Continental ContiTouringContact CT95...$67/ each plus shipping and mounting & balancing...
They have a couple of slightly cheaper ($2-$5 each less) tires, but IMO, significanly lower quality as well...
Since we really have no idea where you live, (profile shows PEI, USA) it's hard to recommend a place to get your tires.