There are many aftermarket PA systems available, cheaper than the factory option too. They are typically tied into the reverse gear signal to activate them and they only beep or have a display via LEDS that is placed in the interior somewhere.
Problem is, the most expensive part is having the sensors that are placed in the rear bumper colour-matched, unless you don't mind having black sensors in your bumper

 

I have navigation/rear view camera, and I still wish I had PA. The rear view camera helps, but it doesn't work miracles. Then again, I have really bad depth perception.

 

Funny,I just came back from Costco and they had a kit at the end of the auto aisle.I didn't notice a price,though.

 

For me, I prefer the rear view camera over PA. With the camera I can see
exactly where I am or if I've crossed a line or not. PA just tells you
how close you are to hitting something.

 

Here's an example of a few systems, many under 200$. There's even one on there with a rear view mirror with integrated video display

Link

 

This is kind of off the subject but the US has signed into law a bill that will require all newly sold vehicles to be equipped with a back-up camera device by 2010. This is just like the TPMS legislation so I would expect the effective date to be pushed back some.

This was done to try to prevent injuries or deaths when kids are run over by a vehicle that is going in reverse. Something like 5000 kids meet there demise is this way. Kind of a bummer.

The sensors alone just are not good enough to detect objects laying on the ground and give a false sense of security to all those people that had to have a big ol’ SUV etc… Even if they name it “Park Assist”, users think it has other capabilities.

I guess my point is that the camera solution, from what I have read, is statistically a better method for rear view systems. The sensors in the front bumper probably are a good solid solution for parking.

Regards,

 

I have the backup camera and find it more useful than the PA system (my cousin doesnt have the nav but does have PA). The PA system you have to trust nothing is there and appromxiate how close you are with the beeping. As for the camera, I can see exactly where I am. If you look closely, you can see your bumper (and license plate too) on the screen an gauge exactly where you are. Probably the most useful option I bought.