I contest The idea that it takes 700mil or 1bil to make a game engine.
I’m not saying I can do it. I’m saying a competent team does not need that.
GTA 5 used an in-house engine (used previously, sure, but StarEngine is also modified from a pre-existing engine). The combined development and marketing budget is US$265 million. Development started in 2008 and released in 2013.
Cyberpunk 2077 also used an in-house engine, budget (not sure if marketing is included or not) is US$316 million. Development started in 2012 and released in 2020.