The cost of constructing Metro increases every time it is re-estimated and that fact has contributed as much to Metro's long-term image problem as anything else.

What started in 1969 as a $2.5 billion system was subsequently re-estimated to cost $2.9 billion, then $4.5 billion, then $5 billion and now $5.6 billion with no significant change in what was to be constructed.

It is generally agreed now that the system will cost about $6 billion, including several million to correct some mistakes made in earlier design and construction. Even that $6 billion figure assumes that Metro's construction program will pick up speed after federal approval of the long-range financial plan that local officials are preparing. The federal budget for fiscal 1979 contains no money for Metro construction for the first time in years.

Delays and Vietnam War-era inflation have been the primary factors in Metro's rising construction estimates, according to Metro's Donald R. O'Hearn, who has been responsible for most of the cost forecasts.

When O'Hearn and his staff first estimated Metro's cost in the late 1960's they built in assumptions that the entire 100-mile system would be completed in about 10 years and that the annual inflation rate would be 3.5 percent.

Almost 10 years have passed and only 25 miles of the system are actually in operation. The annual inflation rate is running at least twice the 3.5 percent predicted and it has been higher than that several years.

Delays in the construction schedule have been forced by Congress, which held up appropriations for two years; by citizens, who thought the subway was a fine idea as long as it did not run under their house; by requirements for environmental impact statements after construction had started; by strikes, and by the court-ordered addition of elevators to make Metro fully accessible to the handicapped.

O'Hearn said that the cost increases that have afflicted Metro are but a reflection of what has happened in construction generally. The prices of some key commodities, such as carbon steel, reinforcing steel and asphalt paving, have more than doubled during the life of Metro construction and have, in fact, outstripped on a percentage basis the Consumer Price Index.

The average hourly wage for a laborer has increased from $3.55 when Metro began to $9.77 today.

Cost overruns on expensive tunneling contracts have run about 10 percent. Social Security has increased dramatically. Workmen's compensation insurance costs have skyrocketed.

Although all that is true, part of Metro's image problem stems from the facts that it was quiet about the inflationary pressures on its longterm budget during the years of heaviest inflation.

No public re-estimate of cost was made from November 1970 until January 1974, a period during which the cost increased from $2.9 billion to $4.5 billion. The cost is now officially re-estimated every six months.

"Metro's construction schedules have always been far too optimistic and have always assumed congressional appropriation levels that simply were not realistic," an important federal official said. "We have yet to see a figure that really represents what we think Metro will cost."

Alan F. Kiepper, the general manager of the Metropolitan Atlanta Rapid Transit Authority (MARTA), where a new subway is scheduled to open in December, has had a similar experience with costs there. Now, he simply refuses to issue estimates on what it will cost to complete MARTA.

"There is no way anybody can say how much anything will cost beyond about 18 months." Kiepper said in an interview last October. "Anybody who tries is just building a rope to hang himself."