For HK you're looking around $79 per tweeter up front ($158) and $79 per tweeter in the back ($158) - $316
For HiFi you're looking ar around $46 per tweeter up front ($92) and $53 per tweeter in the rear ($106) - $198
Those are the prices on RealOEM.com but I am sure you can negtiate with your dealer or one of the many vendors on here, even eBay.
Some people say that you don't need rear tweeters but they do make quite a noticable difference to sound projected within the car. Although not as great of an influence as the front tweeters (for a front passenger/driver) the rear tweeters do a great job of lifting the sound throughout the whole car and you can definitely pick it up, it gives a more surrounded feel. If you have friends in the car a lot they'll surely appreciate the tweeters in the back also.
You'll also need the front speaker grilles/tweeter pods to do this upgrade if you don't have them, they're around $35 each.
If you're really into music then the Alpine upgrade is probably the best. That's around $900 but comes with tweeters, new front mids and an amp. This will boost the sound up front for sure and run more power to your underseat subs You will still need the tweeter pods though at additional cost and this does not include rear tweeters or any rear upgrade...I'm not sure you'd need it though if you rarely have people in the back enjoying music.
You have other options, such as the Blaupunkt THA series amps, these are plug'n'play and work with all existing speakers. The smaller 2 channel THA275 can run from the power of the stereo and power the front speaker circuit (mids & subs), the larger 4/5 channel THA475/575 amps need battery power but can run the rear speakers also. There is one major drawback with these amps though, if you add in tweeters to the front speaker circuit you'll end up overloading the amp anf blowing it so really, it's amp or tweeters, make your choice here. Technically what you could do is disconnect the tweeters from this circuit and make them run directly from the stereo but that involves some splicing of your stock wiring loom. I'd not want to disconnect the mids or subs from the amp as they'll give the greatest benefit and let's be honest, base is what the system lacks at times (although much improved with just a tweeter upgrade as I say).