I would always buy from an authorised Microsoft reseller. The versions you see that are significantly cheaper are most likely illegal keys that have been taken from one of Microsoft’s corporate plans where you get hundreds of activations per key. Just because a key activates doesn’t mean you have a valid license, a valid key does not equal a valid license and if you get audited you will be in trouble.
That being said, I’m not saying the $300 version on Amazon is illegal, but I would be highly suspicious. It may be it is an OEM copy from a server purchase that wasn’t used, although I’m not sure if you are actually allowed to sell a copy that was bought with a server, you would have to read the small print.
If you are buying a server new, then buy it with the license, that will be the cheapest official way. I know it is tempting to save money on the license, but Microsoft has to make money from what they do. If you administer their products then they keep you in a job. If you don’t like what they charge don’t use their products.