The analysis by the NAHT shows that on average councils make annual per capita payments of pound;2,770 per sixth-form pupil, though this represents only about 80 per cent of sixth-form funding.
Government spending plans, meanwhile, show it costs pound;7,380 for a school sixth-former to achieve three A-levels, compared with pound;6,250 at FE colleges and pound;5,910 for sixth-form colleges.
Education Secretary David Blunkett has already guaranteed that sixth-form funding will be safeguarded despite post-16 reforms - but he has said they will be expected to share courses and sites with colleges.
Mr Blunkett has asked why sixth-form colleges produce the same results for less money.
Consultation on sixth-form funding closed last month and decisions are expected soon.
The NAHT says the London borough of Kensington and Chelsea gives its schools the most, at pound;3,316 per sixth-former. Meanest was Bournemouth, at pound;2,008.