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It turns out that convincing billionaires to invest millions in luxury real estate is not the easiest sell.

In big cities across the US, glass skyscrapers and sprawling spec homes that broke ground to fanfare just years ago are now standing with empty units left to sell. Some have said there has been a slower influx of foreign capital thanks to economic instability abroad. Meanwhile, stricter regulations on all-cash, anonymous real-estate purchases — a favorite of foreign investors — have been introduced, potentially throwing some cold water on the luxury real estate boom of recent years.

As demand for multimillion-dollar properties has fallen in recent months, developers are leaning increasingly on flashy extras to attract buyers. Here, we've rounded up some of the most extravagant perks we've seen lately.

Cars

In a similar vein, a luxury home developed on spec by Douglas Elliman's Oren Alexander with his father, Shlomy Alexander, is priced at $36 million — or $43 million if you opt to buy some of the amenities on offer, which include a 1948 Jaguar XK120. The home, in Miami's ritzy Bal Harbour neighborhood, was designed by award-winning architect Chad Oppenheim, who most famously worked on director Michael Bay's mansion in Los Angeles.

Yachts

The Alexanders' Bal Harbour home could also potentially come with a 55-foot VanDutch yacht, should the buyer choose to buy it.

"Our main goals were to simply create the best lifestyle experience possible," architect Oppenheim told Business Insider of the spec home's design and amenities.

Art galleries

Many spec homes also offer the option to buy a curated art collection. For their Bal Harbour spec home, the Alexanders worked with a local Miami gallery to purchase an extensive set of work that includes an Andy Warhol painting called "Unknown Woman."

Designer furniture

Similarly, the Alexanders' Bal Harbour home is said to be outfitted with $1 million's worth of Minotti furniture.