The ending is just stone cold. Literally. Hester has never been very affectionate toward Paul, but by the time of his illness, she seems to become even colder and, as Lawrence describes, her heart "turned actually into a stone."
After we witness Paul's sickness and death, we patiently wait for Hester to finally express some grief or motherly emotion. We want Hester to give Paul what he so desperately craved in life: Her motherly love.
Instead, shrewd old Uncle Oscar gets the last word: At least she's eighty thousand pounds richer, he says. Oh, and while we're at it, he was probably better off dead anyway.
What. Even.
The fact that Uncle Oscar—the wealthy, successful uncle that Paul's parents so envied, the uncle who exploited Paul's fragile mental state to make a few bucks at the race track—gets the last say underscores the tragedy of Paul's death. The desire for wealth tainted Paul's relationship with his mother all his life, and it's this same desire that leads to his untimely death.