And you can buy twenty packs of Marlboros for the price of a single ounce of decent-quality pot. But good luck having a very good time smoking them all.
Not to encourage, or even endorse, illicit drug use (or tobacco smoking), but thats kinda the point: If you need a product to perform a task only it can do the price of inferior substitutes is pretty much irrelevant if they simply can't do it.
Which in the case of the various Kindles is this: They can't run any of the tens of thousands of high-quality Apps that an iPad can. No Garageband; no Flipboard; no RealRacing HD; etc. etc. So its fine (well, barely tolerable) for mundane tasks like web browsing, movie viewing, or book reading. But any - and all - of those tasks are more enjoyable when you've got four times the screen real estate to work with.
You can certainly make an argument for getting a Kindle in addition to an iPad: Reading on the subway or at the beach. But making the argument that the Kindle Fire is substitute for the iPad, with all that the iOS platform offers, is really stretching reality.