The average cost of a Google engineer is at least $500k/year. This includes all direct compensation as well as amortized office costs, perks, meals and so on. I have no official figure on this but the average total comp can be well-established from levels.fyi as well as my own experience (disclaimer: Xoogler).

1000 engineers will give you a burn rate of $500m/year. I guarantee you the head count associated with Fuchsia is higher than this, probably much higher.

Again, I have no official information on the current resource allocation but you can figure these things out by, for example, looking at the leadership structure. At a company like Google, certain positions will indicate head counts. An engineering director probably averages ~100 engineers rolled up through 2-3 layers of managers. A VP means 200-500.

Familiarity with how Google staffs projects, how much Fuchsia was staffed while I was still there and the costs innvolved gets you easily into the billions of dollars over 5+ years.