Jeremy told Julie he paid MM £2000. Julie did not believe him as she knew he did not have that sort of money available.
There is nothing saying Jeremy had huge debts. But he did not have the finances to live the life he wanted. Or escape from 14/17 hour days on the farm. He agreed in court that Neville's will tied him to the farm if he wanted to inherit anything.
He had short term relief when he robbed the caravan site. But for Jeremy that would just pay for a few lavish meals.
There are many other reasons he committed the massacre. Thread already created. However money was the main reason. The judge saying it was a lot more than £436,000.
The relatives were rich in their own right. Source already provided.