If you're looking for an installed version in an older car, you can look at Sony's site - they have a few basic units on Sale in the $100-150 range. If you can live without all of the bells and whistles (i.e. controlling pandora or other music on your phone) you can get a basic unit that offers bluetooth phone and audio capabilities - it will work with both current and older phones. -http://store.sony.com/car-stereo-receivers/cat-27-catid-all-car-stereo-receivers
You can also try crutchfield -
http://www.crutchfield.com/ - they generally tend to be more expensive but definitely talk to their staff (via chat or phone) who are very knowledgeable, and they also typically throw in wiring harnesses and other accessories that you will pay extra for elsewhere (especially useful if you want to do it yourself). They may also have some clearance units that are in the $100-200 range.
I wouldn't recommend buying a new head unit with CarPlay or Android auto just yet, as they're both not quite mature technologies.
One general caveat - Not all head units work with steering wheel controls - make sure you check before buying.