The bad thing about practicing on really cheap knives is that you may never know for sure if your technique is good. Very cheap knives are made (usually) with junk steel. It may never be able to take a decent edge, even if you're doing it right. That makes for a lot of frustration.
You'd be much better off spending that same $25 on one, or maybe two, older used knives from reputable makers (Case, Buck, Ka-bar) whose quality is well established. They've always used decent steel (excellent, in many cases), and their blades respond amazingly well to good sharpening technique. It'll leave no doubt when you get to the point where you're doing it right. You'll be rewarded with a knife that you'll be proud to carry and use and show to your friends. Those eBay 'mall ninja specials' will always look (and probably perform) like cheap knives, regardless of whether they've been sharpened or not. And they're not exactly the practical type of knife you want to carry everywhere and show 'em off, unless you're really into scaring people.
