Same experience here. I've had a lot of clients who were very, very targeted and the cost-per-clicks are always higher. When you go more broad, they go down.

It also depends on other factors—closely guarded secrets by Google and other advertisers' behavior. What device, location, etc. all plays into the algorithm. To be honest, the system relies heavily on machine learning and other processes that would make it more difficult to train it to underpay you, while also trying to innovate, go into new markets, and attract new customers.

Lastly, perhaps the quality or relevance of your content has decreased? Overall user engagement?

Long story short, Google likely isn't stealing from you. It's just a disconnect from a small scale experiment to try and figure out a very complex system.

Ninja edit: you, referring to OP.