ON ITS FACE, the financial plan unveiled last week by Metro's board of completing the subway and operating it and the buses seems sound. But the figures are staggering. The total bill for construction the full subway system has leaped to somewhere between $6.7 billion and $6.9 billion. The operating deficit for the subway and bus system in 1990 has been projected at $329 million annually.

We do not intend to get into the details here of how the board hopes to raise that kind of money. At a minimum, new taxes will be required in all local jurisdictions, and substantial changes will be needed in the way the federal government has looked at Metro recently. But in thinking about those new figures, it is making an assumption that the cost of living will increase at the rate of 6.11 percent every year. In other words, that 1990 operating deficit is $172 million in 1978 dollars, not $329 million.

What is most encouraging about the plan is the response it has received from Secretary of Transportation Brock Adams and, apparently, the White House. Secretary Adams had been leaning heavily on local officials to produce just such a proposal. Once they did, he reacted quickly and favorably to its general format, if not to all its details. The full support of the administration will be needed if so ambitious an undertaking is to be approved on Capitol Hill and in Richmond and Annapolis, not to mention in Arlington, Alexandria, Fairfax, Montgomery and Prince George's and at the District Building.

The most important aspect of the plan is that it lays out in great detail the options that are still open to the metropolitan area. The costs of finishing a superior public transportation system are huge. But so are the costs, especially in social and environmental terms, of not finishing it. That will have to be weighed by public officials and voters throughout the area, as will the possible ways in which additional money can be raised.

In fact, the area is confronted with a situation much like the one it faced a decade ago when Metro was born. All the principal local jurisdictions, as well as two state governments and Congress, must agree before this plan can go forward. Because of the size of the financial commitments involved, each jurisdiction will want to give it, and the alternatives, careful consideration. We suspect that when each does, it will decide that this proposal, or some variation quite close to it, is the best course to follow. But we, too, want to reserve final judgment until we have had an opportunity to study the plan in great detail.