Every single bottle of wine is good value*.
*this depends on what you value.
Fortunately for you people who value abysmal wines that are either so acidic you can't taste anything for a week or so dull that you may as well be drinking scented water, there are hundreds of bottles of wine that, for you, are absolutely fantastic value.
For the rest of us, it's a bit trickier.
Wines under five dollars are always, always bad. You wouldn't buy a sandwich for 30 cents (hint: rotten tomatoes, stale bread, and mouldy mayonnaise) and you shouldn't buy $5 wine for the same reason- you just can't afford to make it without cutting some seriously questionable corners. One such corner is known in the industry as the 'second bladder' technique: wineries replace the urinal in the local pub with a large vat, and collect the rather wafty micturitions of the local winos. This pungeant orange soup is dyed red and bottled. It is then sold for less than five dollars.
Wines under ten dollars can be alright. You do get the occassionl corner cutters who overprice their (already overpriced) tepid swill in the hope that those avoiding the $5 barrier will fall victim to their trap instead, but for the most part a $10 price tag means they've avoided the bottom of the barrel and have put at least some thought into it. These wines wont be amazing. The whites will not taste like urine, but they will probably be either quite sweet or a little too acidic. The reds will claw at your throat a little when you swallow, but for the most part they'll be kind to you.
The ten to twenty dollar mark is where things get interesting. Towards the top end of this price range, you might start to get some quality wine making. For example, you might get hand picked grapes, which result in wine that is made from (wait for it)- grapes. Cheaper wines that are machine picked result in wines that are made out of grapes, and twigs, and dirt, and any unfortunate grubs that were around the vines. If you're strictly vegetarian, you really shouldn't buy machine picked wine. If you're strictly into wine that tastes good, you shouldn't buy machine picked either. Wines under $20 will probably be less than a couple of years old, and are generally best drunk sooner rather than later. Unfortunately, it's possible to justify charging $20 for wine that is still pretty average. You can buy new French oak barrels every year (which cost absurd amounts of money) and still make bad wine. Your best bet here is to either taste at the cellar door or get to know your local bottle shop owner well.
Between twenty and forty dollars, you should be able to assume that the grapes have been treated well. You could also reasonably expect to see wines from single vineyards (which means it's better at expressing that particular year and grape as it's not made up of a pile of different grapes), and perhaps some that have been kept for a few years before they were released. If the world were just and fair, it would be impossible to buy a bad wine over twenty dollars. Unfortunately, however, the world in which we live is one where being confined to bed is a better excuse for missing work than going hiking in the sun; where spending more than an hour enjoying yourself a day is considered a luxury; where it's cheaper to order pizza than it is to make it yourself. In this world, then, you can still find questionable wines over $20. How to avoid them? Firstly, steer clear of the big names. Hardy's, Yellow Tail, and (god forbid) Jacob's Creek are sure fire ways to waste money. They, the Masters of Monotony, the Barons of Bland, aim to make every vintage taste just like the last. The wine isn't bad so much as it is boring. Shoot instead for someone smaller who doesn't have to please the plebs, but who can vary each vintage according to the conditions. You might find a few duds, but it's far more likely that you'll find a gem along the way.
When you get above forty dollars a bottle you're well into the territory where the price of production is overtaken by prestige. A bottle of Grange (~$500) doesn't actually cost all that much more to make than the spectacular Kilikanoon shiraz ($40), but you're paying for the brand, for the recognition, and for the fact that it's a bottle of bloody Grange! If you poured it into any other bottle, you could happily sell it for $50, but you'd struggle to get more than that. And hey, if you want the outrageous extravagance of a Grange, go for it. If we're just talking wine quality though, then you're getting ripped off just as much if you'd bought a $5 bottle of swill. Far better to get a dozen $40 bottles, which may not raise the eyebrows in the same way as a famous label, but will be amazing wine nonetheless.