The economics of high-end prostitutes.
Unlike their low-end counterparts, high-end call girls are expected to supply some level of companionship, and often accompany clients to dinners or parties. Because a beautiful and intelligent woman inevitably has other job (and marriage) options, a very high wage is necessary to encourage them to forgo other opportunities, and risk arrest, disease and shame.
And escorts must spend a great deal maintaining their value without immediate compensation. Much time and money is spent on grooming: hair removal, expensive hair-cuts (one stylist I spoke to claims several of his clients are escorts, who spend at least $1,000 a month on extensions and colour) and regular exercise. Many women have had plastic surgery (particularly if they were once men) and maintain an expensive designer wardrobe. Frequent visits to the doctor are necessary to protect against sexually-transmitted diseases.