you can google the parts mentioned and see what value they were new. this will give you an idea of the price, but generally well sorted DH bike cost alot partwise to put together. Check the frame for structural damages, cracks, bent etc, suspension/shocks any squeak, does it have a smooth travel, casette for wear to the teeth, loose chain, condition of the brake disk, pads, spin the wheels to see if straight. take for a spin. Listen for noises, go trough all gears, test brakes. If he doesn't service himself, receipts from shops. there are only two places here in clonmel so easy to check if bike was there.