Question about writing a formula

Hello All,

I am not sure if this is the correct forum to ask this questions, but I'm a little stuck. There might be a simple solution, but it''s certainly skipping me.

Basically, for all our investments, we create valuation records with quarter end date (3/31, 6/30, etc.) with a new value, whether higher or lower in SalesForce. For example, the object contains fields valuation_date, investment_cost and investment_value. I've queried the data for all valuations with valuation_date>=6/30/2011 in a crosstab. I want to write a formula that will give me the change in the investment_value between the two valuation dates and this is where I'm lost.

Any insight is much appreciated. Thanks.