Hi Tia,
To answer your question, no, no one can tell you the average amount that is tipped. All each person can do is share what amount, when, and how they tip. However that information is 100% irrelevant. Each person tip whatever amount they want to whomever they want at any time they want. Trust me no tip wil go unappreciated.
Whatever value you assign to the service(s) performed by butlers, bartenders, cabin stewards, etc is what -> I <- would suggest tipping.
Some people bribe these rather than tip them by giving them whatever amount of money before any services are performed. As someone who has worked in the food service industry, I find that to be offensive, impudent, and curt. To each his own though and if others feel that is an acceptable practice then more power them.