We've been seeing some fantastic prices on last year's Nexus phone leading up to and following the launch of the 2015 devices. This is almost the cheapest yet: $249.99 gets you the Nexus 6, which is still very much a powerful and desirable phone, from Amazon. This is a new device, with the stock 32GB of storage and either the blue or white color option, plus a standard warranty. It's a full $400 off of the retail price at which the Nexus 6 launched a little over a year ago.
If you need a little more storage, the 64GB model is also available at $299.99, though at the moment it's only being sold in the white version. That's the same $400 discount as the base model, again for a new unit with a full warranty. Both the 32GB model and the white 64GB model are being sold with Amazon's Prime shipping included, which would get it to your mailbox before Christmas if you get in an order today.
Remember that the US version of the Nexus 6 is compatible with all major US carriers, AT&T, Verizon, T-Mobile, Sprint, and US Cellular, not to mention Google's own Project Fi - just drop a nano-SIM card into the slot and it's ready to go. There's no indication that this is a sale or promotional price on Amazon, but stock is likely to run out quickly anyway. Get an order in quickly if you want one.