it can be calculated, but absent making some assumptions, you can't get it exact. You can look at their revenue and total seat miles for Q2 2011, figure it out (or find someone who did it on the web) and then do the math. It will not absent some real work be absolutely correct (as you have a 3mo revenue figure for q2 2011 and monthly PRASM for Q2) but close enough.

But for purposes of a comparative analysis, you can take PRASM and use it for a proxy for the revenue growth figures and apply them. Q2 2011 UAL had (excluding special items) revenue growth of 9.1%, PRASM was up 9% (these are from Q2 2010), and revenue was $9.7B

If I average the PRASM figures for the April-June 2012 period UA is 3.26%, DL is 8.3%, and AA is 9.16%.

Without trying to back out the capacity issue (AA cut more, DL was in the middle, UA slightly less - and this should somewhat reduce these figures) had UA matched DL's PRASM growth it would have had $488M more in revenue. Had they matched AA's PRASM growth they would have had $572M in extra revenue. Again the actual difference is probably somewhat smaller, but as an idea of the magnitude of difference its interesting.