Brake Calipers

[kid_nissan]

Are they expensive?....
When  I am driving i hear a metal on metal scrachting noise

[adey]

This could either be your brakes (in which case some brake anti-squeal should work) or your wheel bearings. If it's the former, just buy some canned anti-squeal and spray it between the caliper and the pad... if it's your wheel bearing(s), then it'll be a little more complicated... and you'd probably have to either replace your hub or go to a mechanic to try and get it properly fixed/replaced.

To chek if your bearings are shot, jack the car up so the wheels are no longer touching the floor, grab each wheel and shake it around (side to side) to see if there's any play. If the wheel shakes around, there's a good chance your bearings are damaged.
However, a little bit of shaking could simply mean some of your bushings or various other suspension components are shot or wearing out, I'm not sure how to tell if it's the bearings or the suspension, though...

[Jeff240sx]

Have you checked you pads?  Nearly all brake pads have a metal "singer" that scratches the rotor to make the audible noise.  That one means you need new pads.
-Jeff