His source is probably a Malcolm Gladwell tech talk [1] It is corroborated by other sites on the Internet [2], though Gladwell frequently gets his information from secondary sources so who knows if these are independent? Based on the history of the project though [3], it seems plausible - the Norden bombsight is basically an analog computer whose development started during WW1, and building a computer with WW1-era technology seems like it could easily cost half as much as building an atom bomb with WW2 technology.

[1] https://www.ted.com/talks/malcolm_gladwell/transcript?langua...

[2] http://warfarehistorynetwork.com/daily/wwii/the-norden-bombs...

[3] https://en.wikipedia.org/wiki/Norden_bombsight