A decent system will run you anywhere from 200 to 400 depending on what you want and if you have a decent head unit already. In my truck I run head unit power to the rear speakers because they are in a less then optimal position for anything but reinforcement but in the door I have them through a 2 channel amp, and I have a single 10 behind my passenger seat. All in all I get a good hit from the sub without being the ass hat at a stop lite that is shaking body panels.
If you don't have a head unit I would recommend something that comes with Bluetooth not only for talking but they also allow you to stream music from your phone. I have an alpine cde-135 and I love it it's easy to use and is very tune able, and from sonic very affordable at 165 with free shipping.
http://www.sonicelectronix.com/item_46565_Alpine-CDE-135BT.html
For your door speakers, your speakers are 4" in front and 6x8 in the rear the selection is pretty wide and they should cost you about 60 for a pair. So we are at now a total of 285 plus tax. This alone will give you a good sounding stereo. An amp that will let you go louder and cleaner, depending on your space needs can vary but here is a good 5 channel that will let you hook up a reasonably powered sub as well for 279 bringing your total to 564.
http://www.sonicelectronix.com/item_34760_Rockford-Fosgate-R600-5.html
A decent shallow mount sub, trust me in a single cab it's much easier to work around, will cost you 100 bucks for a pioneer or 80 for a rock fos. Plus a box you would be lookin at 700 and probably 2 nights of labor for a system you can get loud have good bass and will last you a long long time.