The Pentagon now figures that the Air Force's Maverick air-to-ground missile will cost $1.2 billion more than previously estimated.

The cost overrun for Maverick is contained in the Defense Department's latest public accounting on procurement.

In the three months ended June 30, the Pentagon raised its estimate of the Maverick program's cost from $4.94 billion to $6.17 billion -- the largest increase among the 39 weapons listed in the Pentagon's latest report to Congress on costs.

The Pentagon said the reasons for the cost overrun on the Maverick heat-guided missile include:

* A reduction in the number of missiles to be purchased in fiscal 1983 -- from 2,560 to 1,335. This reduction drives up the unit cost and savings from volume production are lost. The smaller purchase was prompted by a decision to extend the testing of the Maverick until March, 1984, when the missile is to go into mass production if problems encountered to date are solved.

* A higher-than-anticipated inflation rate.

* Higher prices than originally estimated from firms making various parts of the missile.

Critics contend that the Maverick, which is supposed to knock out tanks and other armor, is so flawed it is not worth buying, especially since reliable and effective guns are available for the same purpose. Air Force leaders counter that the missile has performed well in testing and will be a valuable addition.