The timing chain serves to translate crankshaft rotation into camshaft rotation. Camshaft rotation opens and closes the intake and exhaust valves in each cylinder. The timing chain also ensures that the valves open and close in synchronicity with the movement of the pistons, the injection of fuel and the ignition sparks.
If you're replacing it as a precaution or because your tech has noticed excessive chain stretch then figure on mid hundreds for just a new chain or $2000-2500 for a new chain and guide rails. Many techs won't replace just the chain particularly on V8 engines.
If you're replacing it because it broke, it's likely that you'll have to do some work on the valves as well.