I think one reason we don't see too many abductions of wealthy children compared to other socioeconomic groups is that they represent such a small percentage of the population. I realize everyone has a different definition of what a wealthy income is, but only 5% of the country makes over 150k a year.
I think there were a lot more abductions of children from prominent families pre-1960's with Charles Lindbergh, Charley Ross, Marion Parker, Georgia Weckler, Constance Smith, Bobby Franks, Eddie Cudahy, Billy Whitla, George Weyerhaeuer, and Bobby Greenlease. Most of these abductions were with a ransom note, and some of these children were released.