I've said before that I think bike shaped objects (BSOs) can have a legitimate place as entry-level bikes for inexperienced and uncertain new riders—people who simply want to give it a go.
But even I took a deep breath when I saw Aldi's ad for a $79.99 mountain bike this week.
Let's put this in perspective: if you take such a bike to your local bike shop for all but the simplest repair, parts and labour will total to more than the purchase price. And on such a cheap bike, components will be low quality and it may be poorly assembled, so repairs are likely to be frequent.
At this price point, many of these bikes are going to end up on the hard waste pile after the first repair bill. I don't think the manufacture of 'disposable' bikes represents the pinnacle of civilization.
But the quality of the build doesn't just end with repairs—this is a product built to minimum specifications, yet made to look like it can handle rough riding. I doubt that it's up to true MTB conditions and it wouldn't surprise me if it comes with one of those warning stickers that say "Not intended for hard riding". This may not be a huge problem, as the most likely use for these bikes is slow trundling on quiet bike paths. Beyond that kind of gentle riding I don't think I'd be prepared to trust my life, health and safety to such a contraption.
That's not to say that el cheapo bikes can't be made to last and even come out as somewhat cost effective (e.g. see BT Humble's Budget Bicycling), but that takes knowledge and skill not usually available to the 'entry level' rider.
Anyway, if a $80 mountain bike sounds like a true bargain to you, then go ahead and get one. But go in with your eyes open: this bike won't be much fun to ride, will be a pig to maintain, and will end up costing you much more than $80.