The cost of duplicating the fire-ravaged Filene Center at Wolf Trap Farm Park will be at least $17.5 million, more than seven times its original 1971 cost, National Park Service officials said yesterday.

That figure is substantially higher than the $8 million to $12 million estimated by the theater's donor, philanthropist Catherine Filene Shouse, who gave $2.3 million to build the center. Park service spokesman George Berklacy said the new figure reflects "the high cost of construction and just of doing business in today's marketplace."

No decision has been made about the design or construction of a new amphitheater or whether the federal government will contribute money to the rebuilding, park service officials said yesterday.

The amphitheater destroyed in Sunday's fire was built of western cedar wood that had not been treated with fire retardant chemicals. Park service officials said they feared fire retardant would discolor the wood that was specially chosen for its esthetic qualities.

The theater, which was uninsured, was built without a sprinkler system because Wolf Trap officials said they lacked money to install one in 1971. A sprinkler system was being installed at the time of the blaze.

Investigators are still trying to determine the cause of the fire and said they hope to conclude their probe next week.