If you want the filler and you think you can afford the Dragon Boxes but not them and Kai, then you should just get the Dragon Boxes. You'll have everything you do want, and the beauty of owning the DVD's is that you can skip the parts that you don't.
I think Zach Logan explained things very well and very fairly. To help make the money-based decision easier, I'm going to crunch some math.
Basics first. It appears that each Dragon Box contains about 40 episodes or more, approximately (and a spiffy booklet that is nice to have, but I digress). DBZ has 291 episodes. The first two cover up through the 83rd. So, it reasonably follows that there will be seven boxes.
Kai is definitely at least 50 episodes, it could theoretically be about 100 in the long run if it eventually goes to the end. Let's assume it will. We now know that Funimation will release it with the "season x, part y" strategy that they usually use. This likely means that each release will contain 12 or 13 episodes, as this is what it has meant for nearly every recent FUNimation DVD release for a new anime. This means four "seasons" of 25 or 26 episodes each in total, broken up into halves.
Now, a Dragon Box is $60 MSRP. The first release of Kai will be $50 MSRP for each half season box. But MSRP is always discounted somewhere, so let's assume what I'll call a "real world" price: a 25% discount that is, as near as I can tell, a constant when it comes to pre-orders (Dragon Box 2 is 25% at Amazon right now). Finally, there's what I'll call the "ideal price," the 40% price if you wait and get a studio sale at Rightstuf or wait for Amazon to do a sale that offers a discount equal or even greater to that. But let's keep this simple at a flat 40%.
So, MSRP price: $60 for Dragon Box, $50 for a half season of Kai (source: Walmart.com).
Real world price: $45 for Dragon Box, $37.5 for a half season of Kai.
Ideal price: $36 for Dragon Box, $30 for a half season of Kai
Therefore...
Total cost for DBZ Dragon Box: $60 x 7 boxes = $420 MSRP; real world price is $315, ideal price is $252
Total cost for DBZ Kai: $50 ($55 for Blu-Ray) x 8 half season boxes = $400 MSRP ($440 for Blu-Ray); real world price is $300 ($330 for Blu-Ray), ideal price is $240 ($264 for Blu-Ray)
If we happen to break it down to cost per episode... (dividing # of episodes into the price, rounding up)
Dragon Box: $1.44 MSRP, $1.08 real world, $0.87 ideal
DBZ Kai (assuming 104 episodes): $3.85 MSRP; $2.88 real world, $2.31 ideal
This proportion in cost is entirely natural. DBZ is old, Kai is new (after a fashion). The episode counts are also very different, obviously.
Still, the conclusion is evident: if you buy Kai on DVD, do it because you want Kai. Don't do it because you want to save money. You won't really save anything worth mentioning for the complete story unless you wait for Kai to be released in a collection, which almost certainly won't happen until 2011.
So, there you go. I don't think I screwed anything up.
Now, on Blu-Ray, there is a better argument. Funi almost certainly will have the HD masters. Oh, and there are also other factors to consider, such as the redub at a minimum.