[The Ghost Knife]

What's a good low-end Lumia for At&t in the US? And what's the price for it?

[Alienhead95]

I would recommend the Lumia 635. You should be able to get one for around 50 bucks.

[RumoredNow]

That is kind of a loaded question.

At the low end you should probably prioritize a couple of things you must have as it is a game of compromises.

[The Ghost Knife]

Well I want the best most functional low end phone on the market

[xandros9]

That's not much better.

But anyways, I'm going to disregard unlocked phones from my suggestions.

a used ATT Lumia 820 is probably my recommendation.
They're pretty cheap these days, rivaling the 635.

The newer 635 is another good choice.

A used once-high-end Lumia 920 can be a good choice if the price is right.

etc. etc.
It really varies, a price range would help instead of more vagueness if you really want to zero in on a phone. But otherwise looking at those above'll be fine.

[RumoredNow]

Lumia 520 or 521... They are ubiquitous at the low end and synonymous with value.