If you want economical, skip installing rear speakers all together. What I would do....simple, get a 4 channel amp (RF is fine) that does around 50W x 4.

Grab a set of components for the front doors - I think the best deal around right now is the CDT CL-61's over at thezeb.com - 150 bucks for the pair.

Then grab a single 12in sub, and put it in the proper box. For a budget, I really like the IDv3 series from Image Dynamics. Get a D2 so that you can wire it to the amp.

Have the amp run everything, and you will be fine.