I think you´ll find some information in Saif´s Diseases of Poultry (http://www.amazon.de/Diseases-Poultry-Y-M-Saif/dp/0813807182/ref=sr_1_2/278-9441312-5784337?s=books-intl-de&ie=UTF8&qid=1387285124&sr=1-2&keywords=diseases+of+poultry) or in the AAAP Manual on Poultry Diseases. I don´t know if there is a real list on the economic impact of poultry diseases.
For pigs, this list would certainly depend upon the part of the world in which you live, but for the United States, you could get a good idea of high impact diseases by looking of the RFPs from the National Pork Board for past few years: http://www.pork.org/Research/2749/ResearchRFP.aspx#.UsCLo_34hhM
Not listed in the RFPs are PRRSV and PEDV, which are currently two of the highest impact diseases, because they were covered in separate RFPs. As mentioned above, I'm not sure if a specific list of diseases by economic impact would be available as there are so many variables to consider including geography and what exactly you mean by important (actual realized total costs to the industry, costs to individual farms affected versus unaffected, costs if introduced in a naive population, etc.).