There are a lot of factors to consider if you're looking for rates.
I'd recommend contacting multiple ad networks while keeping your current rates and stats (CTR, traffic, etc.) handy so they can give you an accurate estimate of how other clients in your bracket are faring.
That said, there's still more factors that can be on your side as well. If you have a website that appeals to more demographics, it'll be easier to have a higher fill-rate, or a demographic that tends to click more. On that note, you should make sure you have an ad network that optimizes well (each optimize differently).
Another important factor to consider on your side is what ad campaigns an ad network is running. If X network is running a huge campaign for Y company, the Y ads will may take up a lot of the impressions.
If a scenario like that affects your fill rate, this would be a good time to have another ad network that helps leverage whatever happens.