The judge told Matthew Pearce, 22, and Darren Jones, 21, who had denied the murder, that what they had done had horrified all decent people.
Winchester Crown Court was told that Grant Price, 43, who was out shopping with his son David, was seized at a car park in Gosport, Hampshire. He was driven to a shingle spit near Keyhaven, Hampshire, stabbed and left to die.
Mr Justice Ian Kennedy told Pearce and Jones: "The very concept of this murder horrifies all decent people.
"To kidnap a man, to leave a little boy standing there in the street, to cart him about the country and then that long walk up the beach chills the blood. How anyone could be so cruel and heartless as you it is impossible to understand."
Pearce, a store manager, of Portsmouth and Jones, unemployed, of Southsea, Hampshire, were also jailed for 10 years for kidnapping Mr Price, which they admitted, and a further five years for unlawfully wounding James Wink, 20, a student, which they denied.
Jones was also jailed for five years for an armed robbery on a building society. The sentences were consecutive.
The court heard that Mr Price, a father of two, was bundled into his own car, bound and gagged, after being selected at random. He was later marched by his killers one-and-a-half miles along the shingle spit at Hurst Castle to the "spot of his execution", the prosecution said. He was told to get on his knees and stabbed in the back of the neck. He was stabbed twice more, in the angle of the neck and jaw, then in the left cheek.
Partly paralysed from his injuries, the accountant took 24 hours to die from a combination of hypothermia, shock and the wounds. His body was found by a ferryman six days later.
Michael Hubbard QC, for the prosecution, said the accountant must have known that the forced walk along the shingle bank "was going to be as likely as not his last". He told the jury the manner of his death was "as callously cruel and chilling and calculated as one can imagine".
Between his murder and kidnap, Mr Price was driven round in his car while attempts were made - some successful - to use his cash card to obtain money from cash machines.
The prosecution said three hours before his kidnap, and five miles away, the two men attacked Mr Wink, stabbing him through the hand and into his leg when he refused to hand over his car keys at a multi-storey car park at Fareham, Hampshire.
During their trial, Jones and Pearce, who admitted kidnapping Mr Price, each blamed the other for his death.
Jones claimed he waited in the car while Pearce alone took the accountant along the bank, returning to say he had killed him.
Pearce said he thought Mr Price would be put in a boat. Jones led the way along the bank, while he and the accountant followed. He claimed Jones told Mr Price to get on his knees, then stabbed him in the back of the neck and grinned.
"I was terrified," Pearce told the jury. "I had just seen a man killed, a man I had been talking to for a good period of that day, saying it was going to be OK."
After the case, a senior detective on the inquiry said he believed Jones and Pearce had been acting out a fantasy.
Detective Chief Inspector Nick Imber said: "I believe their motive was robbery, and as it went along their fantasies took over. Individually they are violent men in their own right, but put them together and they are a cocktail for disaster."
Matthew Pearce, who in a police bugging operation was heard boasting he was a "hitman", was within weeks of joining the Army. It was suggested during the trial he had wanted to commit a robbery to clear his debts before joining up. Darren Jones had earlier failed in an attempt to join the Royal Marines.
"Imagine two young blokes, macho men, physically geeing each other, and they did fantasise an awful lot. They fancied themselves as being good criminals or hot shots in the Army," said Det Ch Insp Imber.
"The object of kidnapping Mr Price was robbery, but as it went on, their fantasies took over and he paid the ultimate sacrifice of their fantasy."