Yes -  you can create a calculated field with two (or more) custom SQL statements. What exactly are you trying to compare and where is the information stored?  How is it linked together?

As a high level example, I am going to assume that you use a job number to link your estimates and costs together (and as such I would use AR as your primary table).  I will need create one SQL statement to pull the bid prices.  I am going to assume that the bid prices are stored in 6-2 Budgets: [SELECT SUM bdglin.ttlbdg FROM bdglin WHERE bdglin.recnum = {actrec.recnum}]. Secondly, I would need to pull the job costs: [SELECT SUM jobcst.cstamt FROM jobcst WHERE jobcst.jobnum = {actrec.recnum}]. I can then create a third calculation which shows the difference of these  [SELECT SUM bdglin.ttlbdg FROM bdglin WHERE bdglin.recnum = {actrec.recnum}] - [SELECT SUM jobcst.cstamt FROM jobcst WHERE jobcst.jobnum = {actrec.recnum}]

Tim - thank you! This is still and open question for us and I appreciate your help!

Question: can I create these SQL statements right in the Sage interface or do I need to work outside of Sage with a SQL manager? If so, can you recommend a tool for doing this? I notice you are using the SQL field names - that makes me think you are working outside of the Sage application to work directly with the database. You might be interested to know that we are using FileMaker Pro to read (ODBC) the Sage SQL database - it allows us to search, bid and query in a way that Sage does not. That said, I am interested to know what tool you use to create the query you describe. 

I am doing this all through the SAGE interface in 13-3 Report Writer.  Each SQL statement is defined as a a Calculated Field. 

Thank you again, Tim. Very helpful.