Agree that gifts are far more personal, but I tip in the $100-$200USD range. One Concierge, in particular, organises all of our trains and vehicles in Egypt. He knows our preferences and the entire experience is seamless from one FS to another. He coordinates with the other Concierges so that picnic boxes are packed and we are met immediately outside our train carriage.

The most exceptional experience was when I wanted a pair of Gucci trainers and he had the luxury department store photograph their collection and emailed it to me with the prices. When they didn't have the particular model I wanted, he ordered them from Gucci.com and had them delivered to my home before I finished my vacation.