It depends entirely on the niche, and how you monetize the traffic, but I can give you some real world examples:
1: One of my sites used to get around 500,000 Uniques per month, (in the MMO niche) and it made me around $50k per month.
2: Another site I built (web hosting niche), used to get around 135,000 uniques and generated around $80k per month.
3: Another site I bought once (funny viral photos and stuff) was getting around 350,000 unique per month and only made around $1555 per month.
So yeah, it really just depends, but that should give you some idea.