The Parking Sensors option became available for new cars in August 2013 for $500 when the car is manufactured and the Tech Package is ordered. Later it became standard. You may be able to retrofit these early cars. The cost depends on if the wiring is in place or not. Most cars above VIN 12142 have the necessary wiring. In addition, the car must have the Tech Package. This is a parts and labor-intensive upgrade as the front and rear bumper fascias and nose cone are all replaced at a minimum, In addition, the bumpers and sensors need to be painted to match the car.
Official pricing is $5,000 for the base parts and installation, but not the bumper painting or taxes. A rough total estimate from Tesla in California is $6200 (April 2014) and requires the car for 3-5 days. This is likely the worst case (i.e. below VIN 12142 without the correct wiring harness). Others have had prices closer to $2500-3000 for the work with cars with a current wiring harness.
Check with a service center if it is still available and for current pricing, which may vary by region.
Tesla Forum: Parking Sensors
We also installed our own third-party parking sensors to our January-2013 Model S, which did not have the wiring harness. Fun instructions here: