It's not slang, per se, rather ねーじゃねーか is rough speech for ないじゃないか.
So it breaks down as
お宝どころか - "let alone treasure" or "far from treasure"
何もない - "nothing at all"
じゃないか - a tag question like "isn't it" that invites agreement, or serves as emphasis.

So the whole thing would be something along the lines of "There's nothing here at all, let alone treasure." or "Forget treasure, there's nothing in here at all, is there?" The ない becoming ねー simply indicate speaking in a rough manner, and how that might be reflected in the translation is probably best based on how the character's speech is translated as a whole. One that uses British colloquialisms might use, "innit?" Light profanity (e.g., "not a damn thing") might also be appropriate.