This is one of those problems with clients who never tried to rewrite an article, they constantly think that a VA spends less time on a rewrite then on a regular one.
Just an example, a trained writer knows how to write on most topics and can produce his own content out of his head in less then 30 minutes. The good thing about that is that you are giving him freedom and he can use the words which he likes and can form the article however he wants. In most cases he can provide you with an unique article faster then with a rewrite.
Why is that? It is because when you give him an article for a rewrite, then you are actually telling him "look, I want it like this, the same article, different words, has to pass copyscape 100%".
Maybe you think that it's easy, but it isn't. The VA has to figure out other words, rearrange the sentence flow, and, in the end, he can't use his own style. In most cases the article does not pass copyscape 100% (around 80%, because it's a rewrite), so you get a nagging client who want's a 100% passable article, so the VA has to again work his way out and rewrite the remaining articles.
So, a good quality rewrite can't cost less then a normal article, it can cost only more.