I’m assuming that material costs are currently displayed in AU$
In the final game will we be able to select other currencies based on an appropriate exchange rate?
Also how will cost be affected over time? Inflation? tech
Becoming cheaper ad it gets older, more common, etc?
Material costs are displayed in money units, no real currency behind it, because right now it is completely arbitrary. You can see the constant nature of the money unit right now by comparing prices between 1946 and 2020 engines: very little difference!
For the first implementation of the campaign, we’ll probably just go with one singular currency that depreciates over time. If we can make it work and play well, I’d like to see currencies and currency markets in the game that are entangled with commodity markets like in the real world. If that is any fun remains to be seen 
I had noticed since posting that cost didn’t vary too much over time, the reason I had asked was I had no clue if the engine I’d made was ridiculously expensive and priced the car out of it’s market.
Maybe a single currency would be best, have a game specific one like the Sims spend simoleans, that way if multiplayer mode takes off, we are all using a common unit and can compare cost/prices easily without having load any conversion process to the game, Automoleans perhaps 
Not so sure about playing the currency market though, maybe that could be a difficulty option like the stock market is in RR tycoon.