Report that shows all expenses of a job, broken down by category (Labor, Equipment, materials, etc.) and grouped by month

The best way is to create a summary report off the most detailed level of data.  That way you can drill into more detail.

Make an unfiltered report and then make a Report Link field type on the Job to link [Record ID#] to the lookup of the lookup of the [Record ID#] of the job on the most detailed level. In other words, lookup the [Record ID#] of the job down and down to the lowest level of detail and use that field on the right a side of your report link field type.  Then put that unfiltered summary report as the report to use on the form for that Report Link field.  The Report link field will filter for just the job you are viewing.