Parking firms here get an estimated $150,000 a month -- or about 25 cents of every dollar Uncle Sam collects from employes -- to manage federally owned lots at or near government buildings.

There are about 20,000 such parking spaces here, many managed by private firms under contracts awarded on a low-bidder basis.

The federal government prints monthly parking permits and decides which employes get them. Private management firms collect fees, validate permits and are supposed to provide protection and maintenance in the lots. In return, they get monthly payments -- which vary widely from agency to agency and lot to lot -- for managing the government parking lots.

The financial bonanza for some parking lot operators is a result of President Carter's directive late last year wiping out free or low-cost office parking for bureaucrats, from Washington to Anchorage and San Diego. o

Under the president's plan -- designed to encourage car-pooling and save energy -- federal employes who had parked free or for token fares began paying half the commercial rate (or a minimum of $12 a month) last year. The action three more business to private parking firms who had complained for years about free or subsidized parking.

Parking fees will double at most lots in the fall of 1981. That is when the full commercial parking rate will be charged government parkers. Most firms will renegotiate contracts at that time, asking more to cover management costs.

Although some agencies and employes were excused from paid parking, because of their location or critical, swing-shift jobs, most people now pay. Congress and congressional staffers, who take up 4,000 or more spaces in and around Capitol Hill, are exempt from the paid parking, although some aides to pay at private lots.

Fees charged downtown are usually double the rates charged civil servants in suburban locations.

Most federal agencies contract individually with private parking firms.And they have negotiated a wide range of fees. For example:

Employes at the Buzzard Point building in Southwest pay $22.50 a month. The company managing the spaces for the FBI, Defense and other customers gets $6.89 a month for each space.

Department of Energy and Defense and General Services Administration parkers pay $30 a month to park. The managment firm get $4.04 for each permit.

At the 12th and C streets SW lot used by Defense, Energy, Treasury, Agriculture and GSA, the monthly fee is $12 and the parking firm handling it gets nearly half -- $12 a month under terms of an old contract.

New Labor Department parkers, who include employes of Justice, HUD, General Accounting Office, Federal Trade Commission and Securities and Exchange Commission, pay $30 a month. The firm managing the lots gets $6.84 for each space.

In return for managing 1,350 federal parking spaces at Federal Triangle, the government allows the parking firm that has the contract to sell about 300 spaces at full commercial rates, according to the GSA.

To earn the monthly fees, contractors are supposed to maintain lots, validate permits, collect fees and turn them over to government agencies or recreation associations. They are also supposed to provide protection and security for cars. They are bonded.

Government agencies and/or the GSA get back 2 to 3 percent of the monthly fee to pay for printing parking permits, and similar amounts to pay part of the salaries of employes involved in the parking program. The rest goes to the agency to pay rents charged by the GSA, or to maintain parking facilities.

Interest in managing government parking lots has jumped since Carter ordered workers to begin paying, or pay more for spaces. Top executives of some parking management firms showed up in person when bids were taken to handle spaces at the New Executive Office Building next door to the White House. A price of $40 was set for parkers, and bids from management firms ranged from $16.50 to a low of $7.20 a month.