I have a specific story that may end up shedding some light on the phenomenon for you (OP), even though it's not about overclocking.
Last year I decided I needed a laptop. I'd never gotten a laptop before, mostly because I couldn't afford it. I still couldn't really afford it, so instead of getting a really really low-end Dell machine for $500 from some obscure web vendor, I went on ebay and got a compaq 2100 with overheating problems and a broken screen for $170. I cleaned it out and put new heat paste on, and no more overheating. Then I looked at the screen. Well, turns out I was in luck: the previous owner hadn't plugged the cable back in properly after taking it apart; the screen wasn't broken at all. Then I got 1 GB of laptop DDR ($130) and replaced the old 256 mb stick, a wireless card ($30), an old harddrive from a previous repair project for a friend, and I was in business. Well whoop-de-do turns out the backlight breaks after a month, and the hard drive corrupts any OS beyond being bootable in less than half that. So $50 and a couple of botched backlight replacements in addition to a $60 hdd later, I get it up and working again.
Total cost? 170+130+30+50+60=$440 for a laptop with no warranty and a lot of old hardware that could fall apart at any moment. It also didn't come with an XP license.
On the other hand, if I had gotten a budget Dell for $500, I would have gotten a new, fresh system with a warranty and some bundled software. What would, strictly speaking, be the better deal? Well... probably the Dell, and I knew that was going to be the case even before I got the old compaq off of ebay. If the hdd and backlight hadn't broken, it would have been considerable cheaper. On the other hand, if the screen had been FUBAR'd, i would have had to shell out $100-150 for a new one. So financially, I arguably ended up paying more for my craptop than I would have for a new low-end machine. But was it worth it? Oh yeah, taking that thing apart was fun!!! I also learned how to replace backlights, which might be useful some day.
Even though this incidence wasn't about overclocking, it's essentially the same phenomenon. Spending time and money on messing around with your hardware isn't really about money in the end - you do it because you think it's fun, and if that's not your motivation you should really stay away from those voltage settings in the BIOS.