I'm a newbie as well Gokanma, so I can't really say how much of a drop in quality you get from buying an XM or Prime 1 Studio statue. I can give you my observations, about how these statues tend to sell though.
Take for example, XM Studios, the White Queen.
https://www.xm-studios.com/products/white-queen.aspx
If order to have a chance of getting one, you had to pre-order with in the first couple of days of the piece being announce. XM Studios pieces sell out really fast. (The longest I've seen a piece I was interested in, available on there website was maybe 2 weeks) I've looked at a few, but I just couldn't justify the cost.
Prime-One Statues tend to be a bit more laid back selling wise, but it's still very possible for a popular piece to disappear before production is completed.