A copy of a spell (such as a copy of Bring to Light) has no mana spent to cast it (under C.R. 706.10, "a copy of a spell isn't cast"). And in general, a creature, instant, or sorcery card in your library without a mana cost has a converted mana cost of 0 (C.R. 202.3a). Not only can that card be searched for and exiled with Bring to Light (whether Bring to Light is a copy or otherwise, since a converted mana cost of 0 is "less than or equal to" 0 or greater), but that card can also be cast with that spell — since it's cast "without paying its mana cost", which such a card doesn't have and which therefore is unpayable (C.R. 202.1b, 117.6, 117.9, 117.9c) — with the exception of cards that are also land cards (C.R. 305.9).
EDIT (Jul. 1): Clarification.