Come on guys, see the con ! - "haters gonna hate" but let's take this apart anyway... Note, I'm talking about the Pi1 model B here.
To use it properly you will need to buy a monitor. - you can use a TV but it isn't so nice. However, you can buy HDMI to VGA adaptors for about €10 off eBay which opens up a world of cheap second hand analogue displays. Mine is running a 1280x1024 flat panel that cost me €5.
You will need to buy a keyboard - £8 from a major supermarket. Price in £ as a friend got it to save me the hassle of the horrid AZERTY layout.
You will need to buy a mouse. - I rescued an older style optical mouse from a bin at work. Let's just say there was an incident involving coffee. A bit of TLC later, it works fine. If I had to put a price to this, I'd suggest looking at basic keyboard/mouse combo-packs, I think they start around €15 or so.
You will need to buy an external hard drive - really? I don't have one. Well, I do, but I've never used it with the Pi. I doubt the Pi's USB output is enough to run spinning rust.
You will need to buy a power supply - probably. You can run it from a mobile charger but it might be a bit 'iffy'. However, starting from around €10 you can get multi-output 2A tablet chargers. Enough to run the Pi, the HDMI thingy, and a Vonets WiFi adaptor without grief.
You will probably need to buy a usb drive - possibly, but how many people have USB drives kicking around in a drawer? For instance, when I run the Pi with RaspBMC, it is actually installed via the NOOBS package on a 2GiB micro SD card. The same 2GiB micro SD card that was supplied "as standard" in my phone, and got swapped out for a 32GiB one within the hour of purchase. Lower capacity cards aren't so useful or commonplace these days, so I think there are a fair few lying around unused that could be repurposed.
You will probably need to buy a decent container for it to keep it safe. - I have a cute translucent orange one. About €6 from Amazon. The Pi was easy. The Beagle xM not so much, so I found a tupperware container that was Beagle sized and cut holes in it in the right places...
A usable Raspberry Pi for kids to learn to program on will cost as much as a small netbook computer - adding up the above (using €15 for keyboard/mouse combo), it comes to €46 plus the Pi itself. Say maybe €80 as a ballpark figure to include some variations in price. That's less than a useful budget tablet, and less than a third of the retail price of a netbook.
"(which will come with a guarantee that all the bits will work together or you get it replaced.) - the important thing is to go for cheap generic stuff. When you go for fancy multi-key-rollover keyboards, mice with a dozen extra buttons and built in document scanner, and SD cards that are like 40x turbocharged - that's when you are going to hit weird compatibility problems. The first keyboard I tried was a fancy gamer's keyboard (not my choice!) which just about worked under RISC OS and failed entirely with Linux. The keyboard I use now is the cheapest thing on the shelf and it works perfectly.
A Raspberry Pi will cost more than a android tablet. - if you think you can get anything useful done with an Android tablet running an 800MHz single core processor, with maybe 360MiB RAM, a 0.3 megapixel camera (if you're lucky and have one at all) and a tablet sized 640x480 display (can't you see every pixel at the spec?), then yes. The Pi is more expensive. But let me ask you - do you seriously think you could learn anything on such a device? Programming it would be unpleasant without an SDK and dev suite on something else. Those sorts of tablets are consumption only devices for people too clueless to understand why spending another thirty would have been a better idea. Or for parents to give to stressy children that will probably throw it across the room when it doesn't load Facebook fast enough...
A Raspberry Pi is a toy for hobbyists to play with. - probably, yes. If I had children I would get them into using and understanding it as soon as I could. But I am from a different era. I grew up when games were kind of crappy and you bought them on tape. But the most important thing of all is the fact that you could play something, observe it carefully, and think "I could do better than this!". Doing exactly that is how some of us got our first interest in programming. I wrote a rather nice space invaders clone for the Beeb and I guess I should have sold it instead of just giving it away to friends. I wasn't after making money, I just wanted to prove to myself that I could do a better one than the tape I paid £9,99 for. But look at the bar today. Grand Theft Auto 3 on an ancient PS2 is pretty decent looking. I'm enjoying Fatal Frame (aka Project Zero) on the same PS2. If I had more money, pretty much the only game that has interested me in the last few years is The Last Of Us. Can a modern child look at those sorts of things and think "I could do better?". Don't be ridiculous. So we, as the been-there-done-it adults have to find other ways to interest the younger ones in programming. And I don't mean a never-ending stream of fart apps. Maybe if this BBC santioned device is a basic piece of kit, we can throw away the magical mystical high level rubbish and get right back to the core of programming, what really goes on inside a processor. Because for me, the magic is not inheritance or object orientation, but the fact that I am typing this in a browser on an iPad and you will be reading it somewhere else in the world in a browser on whatever - and all the processor is really capable of is pushing values around memory and performing some fairly basic mathematical operations on those values. From these basic operations, we have created UIs and browsers and cat videos. That, to me, is the magic.