Fraud in financial planning cost Americans a half billion dollars last year, and an equal amount may simply have been misinvested by incompetent planners, the Consumer Federation of America charged yesterday.

"While many planners offer valuable services, this financially attractive field is wide open for exploitation by individuals and firms that either cannot offer or have no interest in offering comprehensive, objective financial advice," said Barbara Roper of the federation.

Her study, "Financial Planning Abuse: A Growing Problem," estimates that between 250,000 and 400,000 people across the country call themselves financial planners. She estimated fraud at at least $500 million in 1986.

There is little regulation of the field and planners are not required to meet any special standard of knowledge, she charged. Only a small percentage have completed the voluntary education programs for specialists in their fields.

Nonetheless, outright fraud by planners is confined to a fringe group, she said, with the more common problems centering on self-interest or simple incompetence.

"We determined that while fraud is a serious problem among financial planners, it is practiced primarily by con men operating on the fringes of this legitimate profession," Roper said.

But self-interest is a more common problem, she said, with many financial planners making much of their income from commissions on products they sell. That situation creates a basic conflict between earning money and giving out objective advice to customers. Many, but not all, manage to handle this honestly, she said