Know-How Exchange : Strategy

I work for an educational publishing company. In our industry, the general tendency is to raise most prices every fall. We are always careful to examine competitors' prices and try to position ourselves in the middle of the pack. Our company had a bad year (slightly worse than the rest of the industry), and so our new CEO has asked me to do an analysis to see how past price increases have affected sales.
While I definitely think it's a good idea, I'm stuck as to how to do the analysis. I have plenty of data on sales (thousands of orders a year), but the only software I have at my disposal is Microsoft Excel. So I can certainly see correlation between price increases and sales, but I can't control for other factors (general economic environment, effectiveness of marketing tools and sales reps, etc).
How do I tease out the true effect of price increases? Any pointers? Or am I just making this out to be more difficult than it really is?