Couldn’t easily find an answer, thinking about buying standard. If I enjoy it and want to get the battle chest or eternal edition is that a whole new purchase or just paying the difference from the standard? Not very experienced on how Blizzard does these types of things, thanks in advance for the help 
The only difference between the Eternal Collection and the Battle Chest is the Eternal Collection includes the Necromancer Pack. See my thread here:
You can also download the Starter Edition for free to see if you like the game first.
If you purchase Diablo III and decide to have more content, you will only pay to add Reaper of Souls. This will leave you with the option to add the Necromancer Pack later, if you choose and only pay for that. That’s because your Account will show you already own Diablo III and Reaper of Souls. You won’t have to buy anything a second time.
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Just some more information.
The last promotion sale has ended recently. It was 25% off.
However, usually Blizzard gave 50% off for D3 and RoS; and 33% off for Necro DLC. (Battle Chest Edition and Eternal Collection have the same price as buying individual separately).
I don’t expect another sale soon. So, I recommend you to buy Battle Chest first. Wait for a discount sale to buy the Necro DLC later.
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