Learn German: Niemand erwartete eine so große Abweichung bei den Einnahmen. - Nobody expected such a large deviation in the revenues.

You would change eine so große (such a large) to the comparative eine größere (a larger), and add the subordinate clause: Niemand erwartete eine größere Abweichung bei den Einnahmen, als wir dachten. Remember that the conjugated verb dachten goes to the very end of the als clause.

Why does the German sentence use bei (bei den Einnahmen) when the English translation says 'in the revenues'?

In German, bei is frequently used with statistics, financial figures, or abstract categories to mean 'in the area of' or 'regarding'. Saying in den Einnahmen would sound unnatural, as if the deviation were physically located inside the money itself.

Is there another way to order the words in eine so große (such a large)?

Yes, you will often hear so eine große Abweichung, especially in conversational German. Eine so große is slightly more formal and elegant, which fits the business context of revenues. You could also use the word solche, as in eine solche große Abweichung.

Where does the noun die Abweichung come from?