WASHINGTON, Aug. 18 — President Bush and Congress have agreed to spend $400 billion on prescription drugs for the elderly over 10 years. But they rarely address a basic question: Where does the money come from?

It will be borrowed from the public, officials say. In practice, economists say, workers of the future — children and grandchildren of today's Medicare beneficiaries — will have to pay much of the cost through higher taxes.

The federal government has no budget surplus to pay for the new benefits, which are the biggest expansion of Medicare since its creation in 1965. A law that required Congress to offset the cost of new benefits — either by raising taxes or by cutting other programs — was allowed to expire in September.