<i>but it is usually due to problems regarding budget. Costumers need money in order to be authentic!</i>>
Back on the ancient costume thang - I find the design as a whole for ancient epic is a bizarre combination of wildly off the wall and the anally specific.
I've experienced this at first hand, and it's the result of a total lack of context-awareness (archaeological/cultural) at the production/script development stage, combined with a frenzied late surge on the part of the set-dressing/accessories people to be 'historically accurate'. Usually <i>after</i> they've done those artistic 'concept sketches' for the main characters' costumes.
There's something about people's ideas about the ancient world, which means that costume and settings are no longer 'everyday' - nobody would think of doing an Ancient Roman murder mystery, for example (although they <I>should</i>) - but everyone must look 19th--C-history-painting fantastical.
There is in fact a whole bunch of evidence for Classical and Archaic Greek costume, <i>some</i> for Bronze Age Mycenaean and Not A Lot for Bronze Age Anatolian (which is what Troy should be if they're going the whole legendary hog).
There's also a pile, a <i>PILE</i> of evidence, a lot surviving in colour (some surviving in actual fabric) for ancient Persian costume and jewellery, but very little of it made a strong impression on the look of 'Alexander', for example, because of time pressures.
Basically, what I'm ranting on about here is that ancient costume, beyond what already exists in people's consciousness from Alma Tadema/Last Days of Pompeii silent movie, is barely researched.
And I think Jonathan's right. Films like Troy and Alexander are really in the Lord of the Rings costuming category, rather than the Aviator. They're researched in a totally different way, 'cos the literature's a different style, the sources *look* different and the expectations are different. And thus, 'accuracy' isn't really there in the same way.
OK rant over. Something that gets on my nerves....perfectly servicable source material...mumble mumble....
ps. Did anyone see King Arthur? Kiera's costume aside, I believe they *did* stick everyone, plausibly enough, in late Roman armour - but ironically one person I saw the trailer with hooted in disbelief at seeing the 'plausible' costume:
'Ha ha! why on earth are they dressed as Romans???'
'Er... 'cos...'