Whats the going price for a proffesional musician?
Depends on what he needs to do. A piece for a few synths is far easier to do than a complete -official- orchestration. (with 'official' I mean that it's genuine orchestrated music, not some random synthplayer hitting a few presets from some orchestral expansion board, that'd be the 'few synths'-option again.). Furthermore, most large-scale RPG's are large-scale in everything: a wide range of textures, deep story, much character development, huge maps, and large-scale music. For large scale music you need a solid architecture in the music, which costs time, and is thus more expensive than the odd casual tune one can make in an afternoon.