What was with the fleet of fabulous cars parked on City Hall Plaza Friday? The exotic — and wildly expensive — autos were part of the Gold Rush Rally, an annual cross-country ride that kicked off in Boston this year. There were two Bugattis — each worth more than $3 million — as well as McLarens, Aston Martins, Lamborghinis, Ferraris, and Mercedes-Benzes. The cars, more than 100 in all, with a combined value of $40 million, made a pit stop at Government Center before kicking off the nine-day convoy to Los Angeles. (The itinerary includes stops in DC, Charlotte, Nashville, Colorado Springs, Las Vegas, and LA.) As you might imagine, the owners of such luxury rides tend to be very wealthy. They’re not schlepping across the country. They stay in sweet hotels and dine in fine restaurants. “These people like to party,’’ said Gold Rush Rally publicist Aimee Shackelford. Among those ogling the extravagant rides was 23-year-old Joel Martinez, who was admiring the bright red Bugatti with the HELL BUG license plate. “That car could go 200 miles an hour on the highway,’’ he said. “The cops aren’t gonna be able to stop me. I’ll be playing with them.’’