The Glimpses of the Moon by Edith Wharton

Susy Branch and Nick Lansing are typical Wharton heroes: popular, attractive, and much poorer than their “international set” friends. Like Lily Bart in The House of Mirth, the two depend on the largesse of more privileged acquaintances to get by. Recognizing in each other a desire for the finer things in life, they decide to get married and, knowing that their friends will happily provide fabulous accommodations, live rent-free on an extended honeymoon until either one of them finds a better match—at which point they will amicably divorce and sail off into their separate, wealthier sunsets.
 
But a romantic tour of Europe can confuse even the most mercenary hearts. And when a friend asks for a favor in exchange for the use of her palazzo, Susy and Nick realize that everything in this sophisticated world comes at a price: one that their hearts and consciences may no longer allow them to pay...