It's a skilled profession, but more importantly, it's a particularly demanding profession with very long hours and unique requirements. The wages start in the mid-to-high range of general wages, but get extremely high as you progress upwards. Discretion is what the best butlers are generally paid for, and that always comes at a notorious premium.
I think it's something in the realm of $60,000 yearly equivalent for a decent and accredited butler to work a basic day, without any special requirements like having to answer calls outside of his hours. Keep in mind a basic day is something like twelve hours or so. I think most contracts also include all living expenses covered as a rule, either as live-in or off-site.
A very good butler working in the right household could probably earn enough that their income actually becomes irrelevant, seeing as they only really have to pay for luxuries. There's also the unspoken agreement that a butler in lifetime service will also earn a small percentage of the estate, or in the case of massive estates, a considerable fixed fee, on the death of their employer. That's become much less common in the last few decades, though.