It's quite common. A ghost writer is someone who writes for you, but does not take the credit for his/her writing (though you may choose to acknowledge the ghost writer's contribution).
My suggestion is posting your project on elance.com and guru.com; choose a ghost writer who is experienced (has a number of completed projects and a good rating on them). A
typical rate for a good writer is $50 to $100 per hour (typical productivity: one to three pages/hour according to the EFA resources page).