Well, who's to say that purchased/commissioned cosplays aren't art? It's not just the "costume" that's the art; it's the other factors that come into play. One person could wear the clothing and just appear to be wearing a silly costume; another could wear them and appear to BE the very living incarnation of the character.
If anything, a purchased cosplay is still art; however, it simply decreases the amount of artistry that's attributed to the one wearing it. If the commissioned piece is done well enough, the cosplayer can then focus his/her energies to portraying the character with more accuracy, if they so choose.
Consider Halloween, the biggest night of Masquerades in society today. Whether or not something is totally purchased for $1.50, if the person can utilize that stuff to scare other people to total oblivion, his/her proffession in the art extends further than the individual who simply uses the objects for an inferior result. (I think this idea was brought up before me... -_-)
And, Visual Kei is much like Hair Metal in America: It's a combination of arts, visual and musical. The music of a visual kei band is still "Visual Kei", and it has its own specific sound that distinguishes it from other subgenres of rock. The appearance of Visual Kei is still Visual Kei, as well. Simulating the appearance would still be Cosplay; provided, of course, the cosplayer reflected the character accurately.
Basically, I see the "art" of Cosplay as two things:
*Visual Appearance and generation
*Character Reflection
By "Generation", I mean its creation; how the cosplayer interprets the appearance of the character. If the piece is commissioned, the former point is lost, but the latter is not. If the cosplayer loses the latter, however, the whole point of it is lost, and the cosplay itself becomes a costume as high/low quality as the work put into it.
...I'm reading pretty deep into something I hardly do... *sigh* if I have no place in talking about this, I'll shut up.