Those are amazingly low prices. I don't know how a contractor can stay in business by selling that low. I guess they know better than me what their work is worth. As ranger said, prices have gone up within that past three months, I'd say by an average of 4% - 5%.
Just do your due diligence (license check, references, insurance confirmation, etc....) and then keep control of the money for as long as possible. Don't pay the balance until the job is 100% complete and inspect each window to make sure the manufacturer's labels indicate the window they're installing matches what you ordered.
Sunrise makes a very good window, and the $500.00 rebate is legit.
Good luck to you.