It makes a BIG difference if you are talking about limited series production cars or one-offs.
It also makes a difference if you are talking about a specific car model or a specific serial number car.
However, in general, among pre-WW2 cars the Bugatti Type 41 "Royale" is up there. 6 of these were built (all unique), with some going for as high as a reported $20M at auction.
A Bugatti Type 57SC "Atlantic" went for $30-40M in 2010.
Among post WW2 performance cars: I can't imagine what Ford GT40P serial number 1075 would go for if it ever got on the market.
Modern cars: The one-off Ferrari 612 P4/5 (which I actually saw on the road, photographed, and posted about on this forum) reportedly cost the owner $4M.
The Bugatti Veyron Super Sport in various special edition versions costs reportedly cost around $3.4M.
High-end collector cars regularly sell for more than a Bugatti Veyron:
Most Expensive Cars Sold At Auction - Business Insider
Btw, I really can't wrap my brain around paying aircraft (even business jet $) prices for a single car. For a LOT less I could put together my dream collection of pristine classic muscle, sports, and luxury cars.