It's already been mentioned on this thread...it's the3M-50. I don't believe it's available anymore as it's been replaced by a different model.
Not sure if it can be controlled by an iphone as I don't have one but I can control it using an app on my android phone.
The only thing my $50 programmable thermostat does not have are the sensors, which from my reading of reviews is not very effective.
I'm guessing these sensors would only work properly for one person living in a studio (or some kind of single room accommodation along with the thermostat)
The sensors are a little quirky but for the most part work just fine. So share with us the <$50 thermostat that does everything the nest does, short of the sensors. That's a great price. Geez, you can control it with your iphone and everything?! That'll be front page for sure!