Codex vs Chapter Approved
The codex is law.

It's expected you follow the codex if it's available, otherwise the Index (with many races on it), otherwise the rulebook (if datas specific to an army are in it), otherwise previous codex, rinse and repeat.

Chapter approved are, like erratas, supplementary materials aimed to balance the game. Sadly, Chapter approved can be outdated. If your codex/index is more recent than the Chapter approved, the codex's price is the right one, otherwise it's Chapter approved.

If you do not own the Chapter approved and play in tournaments or whatever, they should be able to give you the correct prices for the units you want to play.

About the "GW are schmucks", they are a business company trying to make money out of your hobby. To do so they have to make it balanced (not funny when someone stomp you because his army is strictly better than your army), they have to make new releases often, and a lot of other things.

By releasing a lot of new things, they have to make sure it's balanced, why not make profits of it?

Also, Chapter approved ends up in erratas and in reprint, usually.

EDIT : Death Guard Codex released in September 2017, Chapter Approved released in December 2017, therefore Chapter Approved's rules should be used, if any. In your example the price tag should be 100. I also went to my local store and they assured me that if it's printed in your Codex and you do not own chapter approved, you can use your Codex's pricetag.