The multi-millionaire singer has accused his accountants, Pricewaterhouse Coopers, and former agent Andrew Haydon, of negligence in managing his affairs.
Mr Haydon was managing director of John Reid Enterprises (JREL), Sir Elton's former management company.
The singer claims that he discovered an alleged £20m shortfall in his finances after he parted company with Mr Reid.
The accountants' lawyers told a preliminary hearing in December that Sir Elton had already accepted $5m from Mr Reid in settlement of his claims against him.
Mr Reid's company, JREL, earned £73.5m between 1980 and 1998, said Mr Pollock.
Sir Elton had an "aversion to men in suits whether they were lawyers, accountants or others badgering him to take on board lots of boring details", he said.
But he had trusted them implicitly and paid them lots of money, Pollock added.
Tour costs
The central dispute was whether or not the costs borne by Sir Elton John's companies "should have been borne by JREL", Mr Pollock told the court.
The singer is claiming that Mr Haydon was negligent in allowing JREL to charge US tour expenses
Mr Haydon has claimed it is normal practice in the music industry.
Sir Elton says a 1984 agreement radically changed the way his affairs were managed.
"This new agreement involved a substantial boost to Mr Reid's income and in return, we say, Mr Reid would be undertaking two new responsibilities," Mr Pollock said.
Audit
The first involved "personal administrative responsibilities", while the second involved moving the burden of various touring costs from Sir Elton to Mr Reid, he said.
Mr Pollock said that between 1984 and 1998, after an audit of the singer's accounts, Sir Elton's companies "bore several million pounds worth of costs or fees which we say contractually should have fallen on JREL".
The High Court action is being brought both by the singer and his three companies W Bong Ltd, J Bondi Ltd and Happenstance Ltd and is likely to last up to eight weeks.
It is not known at what stage the singer will give evidence.