Check your local Costco. Amazon right now have them for $99. I'm pretty sure Amazon has them for 0.01 from time to time. Infuse is considered 'old' phone as in it has been out for a while. So, pay anything over $100 is not worth it.
I understand technology moves fast, but the phone has only been out for a few months, and given its screen size and speed, I doubt the price will be coming down anytime soon. I seriously doubt you're going to find a new one for that cheap, as they're still $100-$200new (with contract) from most retailers.