The most expensive car is the Koenigsegg Agera R which is available for 1,107,000$ from the Los Angeles Car Dealer.
Simply drive, fly(*) or grab a train to Los Angeles. Once there visit the Car Dealer which is a little to the South East of the airport and press
to take a FREE test drive. The achievement should pop as soon as you start your test laps.
(*) I chose to fly which cost about 900$ from New York.