Arthur Shaw says he paid a million for the car but wouldn't sell it for $10 million now. It's probably worth that now. Aren't you losing the original value of the car by turning it into gold. I mean I know $45 million is more than $10 million but you could have turned your bed or desk into gold and kept the value of your priceless car.
VAZ
The actual car he bought may actually just be elsewhere and the purchase of it just a ploy (or what later became the guise) to hide the money as gold. The entire car is a replica made from gold--not just the body but parts of the car as shown at the end of the film (being sent out to people that lost their money).
In other words that isn't the priceless McQueen car he states it is, it's just a replica. Similarly in real life, a real Ferrari 250 GT was not used, instead they had a few replicas made at much lower cost. Obviously they weren't made of gold but the idea is the same--commission the production of a replica using gold instead of steel--and that's what the movie character essentially did.