During 12 years in the Senate, Vice President-elect Walter E. Mondale (D-Minn.) was known as a man of relatively modest financial means.

Many other senators are millionaires, but Mondale lived on his Senate salary, which was $30,000 a year when he first took office but had gone up to $44,600 in 1975.

He also made extra money speaking to clubs, citizens groups and public organizations - $19,426 in 1973, $14,350 in 1975. His wife, Joan, occasionally supplemented the family income by lecturing.

Rooms on the third floor and basement of his rambling, old house in Cleveland Park were rented to students. A student still lives on the third floor, thought the basement has been taken over by Secret Service agents detailed to protect the Vice President-elect.

Last September, Mondale put out a financial statement showing his net worth as $77,361, chiefly, aides said, the value of equity on his home.

Not poor by any means; but not rich, either.

Now, when he assumes office in two weeks as Vice President, his financial picture is going to improve radically.

As one aide put it with a smile, "It's a pretty good thing to be Vice President."

The Senate Appropriations Commitee and financial offices of the Senate have prepared charts showing just how much money flows to the Vice President or is spent on his behalf. The total comes to over $2 million annually without even including Secret Service protection, Navy stewards and groundskeepers for his official residence, free air transportation and other smaller items.

Of course, Mondale didn't run for Vice President just for the perquisites. "But they're mighty nice to have," as one Senate expert said.

First on the list are things provided to the Vice President in his capacity as president of the Senate. They come from the Senate budget.

To start, the Vice President gets a salary of $85,600, compared with the $44,500 received by a member of Congress. The federal pay commission has recommended a raise to $80,000. But even at $65,600, MOndale is better off than a senator - not an unimportant thing for a man with three children and no great wealth.

Moreover, even if the salary is not raised and assuming Mondale serves eight years as Vice President, he would then become eligible for a life-time pension of $36,000 a year based on his 12 years in the Senate, eight as Vice President and two years of military service.

If he hadn't been elected Vice President but stayed in the Senate for eight more years and then lost his eat, his pension then would be about $24,000 because it is computed as a percentage of highest average salary during any three-year period. As Vice President his highest three-year salary average will be far higher than as a senator.

The higher salary and future higher pension is only one new financial advantage.

For his senate domain, the Vice President also gets a $10,000 expense allowance, and about $654,700 for hiring personnel. As a Minnesota senator, Mondale got $493,485 for clerkhire.

The clerk-hire allowance of the Vice President used to be proportionately lower in relation to that of a senator from Minnesota, but when Gerald R. Ford became Vice President, some Republicans argued that he ought to get an allowance at least as high as a senator from a big state like Michigan, so the Senate pegged it as about the same levels as a Michigan senator's.

Mondale can use the clerk-hire money only for hiring Senate staff; it gives big leverage, however. It is enough to hire 20 to 30 people at pretty good salaries, many of whom, if past practices are followed, would be legislative advisers, but others essentially would be political operatives.

As his Senate legislative aide, Monday is expected to hire William Smith, an attorney who worked for Mondale a few years ago when he headed a special Senate committee on education.

In the Senate, Mondale will also get the franking (free mailing) privileges for official letters, an annual stationery allowance of $4,500, a $1,215 postage allowance (for special delivery and the like).

He will enjoy a five-room office suite on the second floor of the Dirksen Office Building, the ceremonial office in the Senate chamber's lobby, plus a two-room office across from the lobby.

Paid for by the Senate sergeant-at-arms and not charged directly as part of the Vice President's accounts will be a leased limousine and a driver (at an annual cost of about $17,000). All furniture and equipment, and local and long-distance and telegraph charges for business.

In addition to these Senate perks, the Vice President gets:

Free residential use of the vice presidential mansion, located at the Naval Observatory grounds on Massachusetts Avenue, plus $61,000 to run the house, plus stewards and groundskeepers from the Navy.

Free transportation in Air Force planes when he must travel, and the definition of "must" is usually rather broad.

A suite of offices in the Executive Office Building which aides said consists of about 25 rooms, and an allowance of $1.2 million to run and staff them, an operation separate from his Senate staff.

About two-thirds of the $1.2 million is to pay staff members in the EOB, consultants whom he wishes to engage from time to time, or personnel he may borrow from other federal agencies.

The rest of the $1.2 million is for office equipment, staff travel, printing and paper and upkeep and costs of the EOB office, for which he must pay "rent" of about $175,000 to the government.

The top aides on his EOB staff - his executive "hat" in contrast to his Senate "hat," will be Dick Moe, Mike Berman and Jim Johnson, all former aides, with Moe probably the chief of staff at $44,600 a year.

Of course, compared to the personal wealth of the current Vice President, Nelson A. Rockfeller, all these perks and benefits aren't so huge.

But not bad for a minister's son from Elmore, Minn.