He was cooked by a Coke. Jurors in the murder...

Princeton grad Thomas Gilbert Jr. was sentenced to 30 years to life in prison Friday for killing his millionaire hedge-fund father over a $100 drop in his allowance — as his mom begged the judge for mercy.

“We were once a very happy family and want to be happy again,” Shelley Gilbert said as she put an unusual twist on her victim’s impact statement in Manhattan Supreme Court by pleading for leniency for the killer.

“We need Tommy to be given as light a sentence as possible,” she told Judge Melissa Jackson. “I ask that you give him a chance and give him hope for his future, not just for him, but for us as well.

“I know if my husband was speaking from heaven, he would be saying the same thing as well.”

A jury convicted the pampered Ivy-leaguer of second-degree murder and weapons charges for blasting his dad, Thomas Gilbert Sr., in the head on Jan. 4, 2015. Earlier in the day, his parents had dropped his allowance from $400 to $300 a month.

“The last thing Thomas Gilbert Sr. ever saw was his own son pressing a .40-caliber semiautomatic pistol into his head and pulling the trigger,” Assistant District Attorney Craig Ortner had told jurors in closing statements.

Gilbert fled, and his mother returned to her apartment minutes later to find her slain husband sprawled on the floor in a pool of blood.

Before the sentencing Friday, Ortner told the court: “This was a cold, calculated, premeditated murder committed by a 30-year-old man.”

Gilbert’s mom, however, told the court that her son was “struck by schizophrenia” and that she “wishes” the judge “and everybody in court” could have known Gilbert before that.

“I know you are not responsible for assigning Tommy to his next location, but I ask you to strongly recommend he be committed to a [psychiatric] hospital close to home so we can visit him regularly,” said Shelley.

“Visits are therapeutic for him and essential for keeping what is left of our family together.”

Shelley went on to say if her family was able to get her son in a psychiatric hospital 15 years ago, “my husband will still be alive today.”

“The state of New York and its lack of adequate treatment for the very mentally ill has caused the agony we have had to endure for a very long time,” she said. “If my schizophrenic son ends up in a prison in upstate New York where we are not able to visit him often that would add to the trauma my daughter and I have to endure.”

Before Shelley gave her statement, prosecutors noted to the judge that the law said victims who are family of the killers are supposed to speak only in support of the victim. But defense lawyer Arnold Levine argued she “knows what the victim’s views would be” and Jackson agreed to let her talk.

Jackson later addressed Gilbert before she handed down the sentence, saying, “You were not insane at the time you committed the crime and killed your father.”

The judge, who told Gilbert, “there’s no doubt that you have mental issues,” added, “you were not insane, and you’re not insane now.”

Gilbert spoke in a rambling manner before the sentencing, referring to himself in the third person.

“Defendant has no criminal record, strong family ties and good behavior while incarcerated. Defendant expresses remorse,” Gilbert said.

Shelley was the prosecution’s reluctant star witness, testifying against her son after she was subpoenaed.

Ortner had argued that Gilbert may have suffered from mental illness but he knew exactly what he was doing when he pulled the trigger.

Gilbert grew up in the lap of luxury, attending the best prep schools, including Deerfield and Buckley, summering in the Hamptons and traveling the world — all on his parents’ dime. He also had a Chelsea rental apartment, a Jeep and two country club memberships — but no job, preferring to live off his parents’ largesse.

At one point, his family was giving him $800 a month in spending money, which was gradually cut in the weeks prior to the murder.

To nudge his son toward self-sufficiency, the elder Gilbert tried to wean him off his allowance and made what would be the last in a series of reductions the morning of the murder.

Ortner had said that Gilbert, driven by greed and rage, “threw the ultimate tantrum.”

Defense lawyer Levine argued that Gilbert suffered from schizophrenia and other mental disorders and that it was psychosis and paranoia, not greed, that drove him to murder.

“You need to consider who Tommy was before,” Levine, who witnessed first-hand Gilbert unravel during outbursts at the Manhattan Supreme Court trial, said Friday.

Outside of court Friday, Shelley told reporters that the family plans on appealing the judge’s sentencing ruling, which she said left her feeling “horrible.”

The mom railed that “the treatment of our mentally ill needs to improve.”