'Big price' is the 72nd commonest 'adj. + price' in BNC (with 9 hits - 22 times fewer than the commonest, that is to say, not common at all ;-)). It is more commmon in COCA - at 22nd (still not very common).
Its use in financial contexts suggests to me that it may have been introduced and spread by non-native MBA students*. I know that doesn't mean it's 'not part of the language', but it explains why emsr2d2 - in common with me and many other native speakers - finds it an uncomfortable collocation.
* Perhaps false friends come into it. Is a prix grand rather than haut, is a precio grande rather than alto, is a preis groβ rather thanhoch...:-? (Those three languages are by no means the only relevant ones - and I have no feel for the strength of the collocations anyway- but they're pretty significant in the development of Am Eng.)
b