The popular "£400 to build the MONIAC" figure is real in LSE's archival record, but prices a different, later machine than the one it's usually attached to — the original 1949 Croydon prototype cost £100

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Nicholas Barr's account, drawing on LSE's institutional archives and James Meade's personal papers, records that after finishing his degree in summer 1949, A. W. Phillips "received the Leeds advance of £100 (about £1250 at today's prices), bought a number of perspex tubes and valves," and built the first prototype in the Langleys' garage in Croydon — funded by £100 from Leeds University, not £400. The £400 figure does appear later in the same archival record, in a different context: a 7 December 1949 letter in which Meade, then LSE Professor of Commerce, proposed how the School (not Leeds) would fund Phillips to build a second, improved machine in 1950: "the total cost of the machine not to exceed £700 ... This would allow £300 for Mr Phillips' salary for six months plus £400 for the cost to him of getting the machine made." Barr further records that the firm actually contracted to build that second machine, White-Ellerton Ltd, "was to be paid £400 ... to cover the cost of materials and his own time."

So a real £400 sits in the primary archival record — but it is the 1949 LSE budget line for the second machine (delivered to LSE in October 1950), not the cost of the original Croydon-garage prototype that popular retellings (Wikipedia, Engineering New Zealand, HandWiki) attach it to. That original prototype's documented cost, per the same source, was £100. See claim-phillips-moniac-hydraulic-economic-computer-1949 for the note this corrects.