It's a good question and encourages a bit of realism.

Putting aside the question of engine & gearbox, where I guess the price range can go from simply transplanting a £500 donor engine all the way up to whatever the cost of a rebuilt Alfa on ITBs is or even whatever a crate or 2nd hand supercharged Evora engine is, where are the other key points for deciding whether you are doing a cheaper or more expensive build?

Paint v no paint I guess?
Upgraded dampers or standard?
Upgraded brakes or standard?
Level of interior trim?
Rally bits or not? (light pod, driving lights, roof scoop, chin spoiler)
Stradale or GP4 body which may influence the instrument cost?
Wheel options?
Tyre options?
Air con or not?
DIY build or factory build?
Others?

I got into this Stratos thing after seeing the Autocar video review where Steve Sutcliffe gave the impression that you could build your own car for about £30k or the factory could build you one for about £40k. That was in 2013. Not sure how realistic that was even back then (genuine question, I don't know) but it was the main thing that drew me in at the time and led me to believe it could be a fairly cheap way of getting a pretty special car. I'm now older, wiser .... and likely to be much poorer. On the flip side, when you see how much demand outstrips supply (3 year wait for a turnkey car) and the consequential robust 2nd hand prices ("kit cars" selling for more than build costs) then it's not exactly a daft way to spend money.