What is the standard cost for the illustration on a Yu-Gi-Oh! card?
It looks like the company that makes Yugioh is very tight lipped about their contracts with their artists.  

More is known about Magic the Gathering artists, they reportedly get paid roughly $400-$600 / card, with "known" artists making more, possibly 2-3 X. 

My guess would be that Yugioh artists make less than Magic the Gathering artists.