Hi Tom and Louis, thanks v much for your responses.
Tom, we are using our own tools to pull from Twitter, it's historical data we are not able to get from the APIs (or services like socialharvest) - but thanks for the tip we will definitely keep an eye on Social Harvest and ways we might be able to use for other problems.
We now have prices back from GNIP, Datasift and others. They start around the $500+ mark / and want $5,000+ for the query we are after - plus most ask a yearly commitment.
Louis, I will be in touch about the NYC agency, would be interesting to get their perspective..