Answer: SAGE

Operational between 1958 and 1983, the Semi-Automatic Ground Environment (SAGE) computer was, and remains, the single most expensive computing system ever constructed. Developed for the United States Air Force by IBM, SAGE was a Cold War-era behemoth designed to autonomously monitor America’s air space for air-based attacks.

While neither IBM nor the government have ever released production costs on the secretive (and now decommissioned) project, civilian estimates put the total development cost at 8-12 billion dollars (65-97 billion dollars when adjusted for inflation). Although it’s important to note that this cost includes the development of the computer system, the supporting equipment, and the bunkers to house it, if the direct computer development cost was even 1/10th of the total project cost, the SAGE computer system would still exceed the expense of any other computer system by a wide margin.

To put it all into perspective, a modern supercomputer like IBM’s Summit (OLCF-4) at the Oak Ridge National Laboratory was developed under a 325 million dollar Department of Energy contract. Realistically, given the decreasing cost of hardware and the increasing power, the chances that a computer project ever rivals the cost of SAGE (bunker or no bunker) is very slim.

In addition to being the priciest computer system ever devised, the SAGE system also takes the prize for the heaviest (a complete AN/FSQ-7 system weighed 250 tons) and the largest (the total operating floor space was 21,780 square feet).