BALTIMORE, AUG. 1 -- The first good news to come out of the new ballpark here won't be a Mickey Tettleton home run or a Jeff Ballard no-hitter. It happened today when bids for a main construction contract came in almost precisely at the estimate.

For a project that already had escalated nearly 70 percent since it was approved by the Maryland General Assembly three years ago, it was good news indeed.

"This was the one that could have broken the camel's back," said Bruce H. Hoffman, executive director of the Maryland Stadium Authority.

Four bidders for the general contract submitted proposals that were within 1 percent of the $45.8 million estimate, a figure that was not released until bids were opened at the authority's Inner Harbor offices a few blocks from the construction site.

Officials said today that construction is on schedule to meet the target date for the Orioles to play their first game in the new stadium on opening day 1992. The $105.4 million stadium, with seating for 47,000, is being built with state bond funds that will be paid off with revenue from special lotteries.

During his first months in office, Gov. William Donald Schaefer persuaded a balky state legislature to approve construction of a baseball stadium and an adjacent football stadium in the Camden Yards area near downtown Baltimore. The football facility will be built only if the city lands a National Football League franchise.

At the time, the cost of the two stadiums was pegged at $201 million, but that estimate was pushed up a year ago to $306 million. The still-unnamed baseball stadium originally was expected to cost $62 million. It was revised to $105.4 million in June last year, setting off howls of outrage from legislators, many of whom opposed construction in the first place.

Planners said the cost overrun for the baseball stadium stemmed from improvements and refinements to the original concept and from unexpected costs in preparing the 85-acre site near Interstate 95.

Throughout the furor, Schaefer remained publicly unfazed by the escalation, but officials were braced for the worst at the bid opening today, remembering the clamor last year when the cost of another Schaefer project, the Baltimore area light-rail line, rose nearly 50 percent.

"We committed to get this done at $105 million," Hoffman said today after the bid opening. "This goes a long way toward fulfilling that."

Contracts totaling nearly $25 million already have been let for excavation, the foundation and some exterior work. Bids opened today were for interior work, including mechanical and electrical systems, the skybox suites, the lounge, the club level and concession spaces. Construction could begin in mid-September, said Kim McCalla, assistant project manager.

Officials said they will decide who should get the contract within two weeks. The decision then requires the approval of the Maryland Stadium Authority and the state Board of Public Works.

The firms' bids: Blake Construction, Washington, $48,877,000; Centex Construction Co. Inc., Merrifield, Va., $45,984,000; Dick Corp., Pittsburgh, $45,938,000; George Hyman Construction Co., Bethesda, $45,742,000; and Omni Construction Inc., Hanover, Md., $45,883,000.