I've been drinking wine for a while, in the range from $4-$100 and even though I wanted to say that the expensive stuff was better, I cannot tell the difference

I've been drinking wine for a while, in the range from $4-$100 and even though I wanted to say that the expensive stuff was better, I cannot tell the difference at all. It all tastes like wine. Every now and then my wife will say that a bottle is "bad" and "vinegar-like" but I still can't tell. I just don't get it. The only thing I can tell is whether a wine is sweet or not - but that only has to do with fermentation not necessarily quality.