DALLAS (AP) _ Federal regulators merged eight insolvent Texas savings and loans Friday, including the biggest first-quarter money loser, into one institution that will be run by the government at a record cost of $2.5 billion.

Federal Home Loan Bank Board spokesman David Colgren said the government estimates it will spend another $3 billion over the next decade, bringing its total infusion to $5.5 billion by 1998.

The merger, outlined by the FHLBB, is the biggest under the government's ''Southwest Plan,'' which is designed to eliminate ailing S&Ls in the region through consolidations. It also respresents the largest bailout in the thrift industry.

The announcement comes a day after federal regulators said they sold 12 insolvent S&Ls in Texas to an investment group headed by William Gibson, a Continental Illinois Corp. vice president. The Federal Savings and Loan Insurance Corp. promised to put up $1.3 billion in government assistance, while the private sector contributed $48 million in cash.

The thrifts combined Thursday form the new institution of American Federal Bank F.S.B., which will be based in Dallas.

In the latest deal, the combined thrift will have assets of $6.9 billion and will be called Sunbelt Savings F.S.B., borrowing its name from Dallas- based Sunbelt Savings Association, which was one of the eight merged thrifts. Sunbelt lost $1.2 billion in the first quarter of this year, a record for the industry.

The new institution will be headed by Thomas J. Wageman, who has been managing the state-chartered Sunbelt Savings Association for the past two years.

The FSLIC, which is providing a $2.5 billion note to eliminate the losses of the consolidated institution, says it plans to shrink Sunbelt with the intention of selling it to investors next year.

''Once we finally implement consolidation and untangle the loan participations, the surviving institutions will be much more attractive to a buyer,'' FHLBB Chairman M. Danny Wall said in a statement.

Besides Sunbelt Savings Association Texas, the other thrifts involved were Western Federal Savings & Loan Association, a Dallas-based S&L which ranked 12th in losses for the first quarter, and Independent American Savings Association of Irving, ranked 20th in first-quarter losses.

The other institutions involved included: First City Savings Association of Irving; Federated Savings & Loan of Brady; Multibanc Savings Association of Alice; Texana Savings & Loan Association, Texarkana; and Summit Savings Association of Dallas.

George Barclay, president of the Federal Home Loan Bank Board of Dallas, said the consolidation is ''another FSLIC initiative to stabilize the thrift industry in Texas,'' which has been hard hit by slumping oil and real estate markets.

The transaction brings to 20 the number of insolvent thrifts in Texas that have been eliminated this week and 32 since the Southwest Plan began three months ago. When the plan began, 109 Texas thrifts were insolvent.

Wall said Thursday the FHLBB expected to complete the plan at the end of the year.