The substantial decline in housing prices that pulled the consumer price index down last month actually is understated, housing industry economists say.

True home prices have been falling steadily for some time--even though statistics have shown them continuing to rise--because of the numerous financing discounts being offered by hard-pressed builders and sellers, according to these experts.

"We have a deflation in prices of homes of a magnitude we have not seen since the 1930s," said Michael Sumichrast, chief economist for the National Association of Home Builders. "We have seen price declines before during recessions, but not to this degree."

The October CPI increase of 0.2 percent was dampened by a 0.7 percent drop in home purchase prices and no change in mortgage financing costs from the month before. The housing component of the CPI showed a zero rise overall, however, because of increases in rents and household furnishings.

From October 1980 to October 1981, home prices rose only 2.6 percent, according to the Commerce Department figures. That compares with a 13 3/4 percent jump from October to October the year earlier, and a 14.74 percent leap the year before that, Sumichrast noted.

Jack Carlson, chief economist at the National Association of Realtors, said that median prices of resold homes declined by $1,900 to $65,200 in October from $67,100 in September, greater than seasonal declines, which usually range from $500 to $1,000.

"Prices have come down more than at any time in the postwar period" during the current housing slump, Carlson said. He estimated that prices of existing homes actually have dropped about 6 percent in the last year when inflation is taken into account.

In addition, the statistics on home-purchases prices do not include the value of discounts that sellers are using to attract buyers in today's moribund market. Mortgage interest is about 17 percent now, but so-called creative financing techniques--being used on about 60 percent of the homes being sold today--can bring rates down to 12 or 13 percent, Carlson estimated.

Theodore Torda, senior Commerce Department economist, agreed that the official price-decline figure is understated because it does not pick up financing concessions. This is one reason why the Bureau of Labor Statistics is changing from a purchase-price figure to a "rental equivalency" measure for housing costs, to minimize the effect of volatile mortgage interest rates on the measurement.

Conventional mortgage interest costs are up 24 percent from a year ago, the Commerce figures show, though they stayed flat in the last month. Interest rates have declined in the last few weeks, however, and this drop will be reflected in next month's figures, Carlson pointed out.

Several analysts are predicting that interest rates will continue to decline and that this will boost the housing market--and home prices -- next year.

Sumichrast said he does not expect a price "explosion" next year even if rates come down, however, because it will take some time for the market to absorb all the unsold homes being offered.

The Realtors' association, meanwhile, reported that sales of existing homes plunged in October to an annual rate of 1.92 million units, the first time in a decade that the figure has gone below 2 million.