As both an admirer and collector of vintage clothing, this is a question that never ceases to get me thinking. I've come to the conclusion that, while the designer undoubtedly plays a huge role in how much an item will sell for, what really matters is if the piece reflects its era or not.
Consider this scenario: If you were given the choice between two dresses from the 1920s, one being a classic flapper shift dress with intricate details, the other a simple jumper that looks like something from Target's latest collection, which are you more likely to pick? Most would probably go for the former, and for good reason--if one wanted a modern look, they would go to, well, Target.
Like I mentioned, though, the name does matter, even if we don't like to admit it. Surely a vintage Emilio Pucci dress from the 1960s is going to sell for more than a no-name dress from the same era, even if they're of a similar style. In fact, looking on eBay, I count at least three Pucci dresses from the sixties--all ending within the next 24 hours--that are going for over $100 each. One has even reached $450 at the time of writing, which is astounding considering it is, admittedly, not the most unique dress I've ever seen.
So, to answer the original question, we have to look at the outstanding trends and silhouettes of this current decade. Maxi dresses and skirts have exploded back onto the fashion scene, so in 20 years, give or take, one in reasonable condition would probably fetch some good cash. That's just one example--this decade is unique in that there is no set style for clothing, and people are given the freedom to wear what they want, so anything goes.