There are places that provide training:
http://www.modernbutlers.com/html/butler-schools.html
Membership in the IIMB, while not mandatory, may be a helpful credential.
"Professional butlers typically earn $50,000 to $100,000 or more per year, plus health insurance, room and board, and use of a luxury car"
http://www.fabjob.com/Butler.asp
I have heard the number is closer to $100K for the truly serious that wants to have a lasting relationship with a consummate professional, though location and demand will be a factor.
I would think in the rarefied world of households requiring a butler, shopping on price is not the best approach.