What are you running for a system? HU, amp(s), sub?
Do you actually want full range out of the back or are you just looking for a little more bass output? Do you have a sub that may interfer with the rear speakers? (i.e. beating the tar out of them) Will these be amped or unamped? Are you willing to buy an amp if it costs, say $80?
A couple affordable options that come to mind is Cadence CWM6 component set and Phoenix Gold's RSD component set. Both of those can be had for under $100. For the rear, it depends... It depends on what you're trying to achieve by running rear speakers. If you want full range for rear passangers and don't mind sacrificing sound quality/staging in the process, fine. If you're just looking for more bass output than the 6.5" woofers will give, then you should look more for a midbass specific woofer rather than a full range one. You can make a wood spacer/mount that will allow you to run anywhere between 6.5" and 8" to maybe even 10" woofers up under the rear deck, however much space you have available underneith widthwise(front to back). This can progress as far as IB subs if you want, a nice route if you don't want to give up trunk space. Again, the choice depends on what you're trying to do.