Your question is a lot like asking, 'How much does a fish cost?"
What kind ( Bass, Trout , or something exotic)
Fresh, or live
Salt water or fresh
rare or common.

Not trying to sound sarcastic, but to get a price you need to provide more info.
Are the piles to be wood, steel, concrete?
What kind of access is available? Can a barge-mounted rig get to all areas, or can atruck-mounted rig get to all areas, or will it require a combination?
Have you had any subsurface work performed?
How do you know that you expect to drive them 40-50'? Is that a SWAG, or local average, or based on some knowledge.

( SWAG= Scientific, Wild-Assed Guess)