This is by no means my area of expertise but I'm currently writing an essay on this very subject, so I'll share my findings.
From what I've found, there isn't really any particular reason why expansion on the Rhine frontier was effectively halted by Varus' defeat. The ancient sources I've looked into don't really explain it too much, but Suetonius' Life of Augustus says that upon hearing of the defeat, Augustus "set up watches throughout Rome" and extended provincial governors' tenures, which suggests that there was fear of uprisings in the provinces as opposed to a German invasion, but nonetheless on the whole Rome took a defensive posture on the northern frontier.
Excursions into Germania did continue but more took the form of large raids, mostly led by Germanicus, and Tacitus' short reference to this argues that these raids were attempts to avenge Varus' legions (and most likely to recover their eagles). An account by H.H. Scullard argues that there were eight legions on the frontier following the defeat, and that this was enough to reconquer at least the lands lost, so why didn't they do so?
C. Wells claims that a number of new Germanic tribes moved into the now empty land that the Romans had left to the east of the Rhine, so not only would they have to deal with their other enemies such as Arminius' Cherusci but also these new tribes if they sought to conquer again, and Scullard believes that Augustus's age and frailty ultimately convinced him to stay put, which is reasonable considering the entrance of new tribes into the equation.
Anyway, why fight when you can get others to do the fighting for you? The Romans would sometimes play a role in placing favourable leaders into positions of power within tribes outside the empire, an act which Augustus himself claimed to be an "example set by our ancestors". It seems that, on the Rhine frontier, the Romans simply switched their policy to focus more on this and making allies with other Germanic tribes. In fact, the Cherusci would later have a king appointed by Rome in Italicus.
I haven't looked into economic reasons but those are my findings. Overall the empire needed to look inwards in case any subjugated peoples got any ideas (as now the myth of the invincible Roman army had been dispelled), Augustus feared further losses on such a scale and opportunities to make alliances with some Germanic tribes made expansion less of a priority.
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Sources:
Augustus, Res Gestae.
Tacitus, Annals.
Suetonius, Life of Augustus.
Scullard, H.H., (1982) From the Gracchi to Nero. 5th edn. London and New York: Methuen.
Syme, R. and Collingwood, R.G., (1952) 'The Northern Frontiers from Tiberius to Nero', in Cook, S.A., Adcock, F.E. and Charlesworth, M.P. (eds.) The Cambridge Ancient History: Vol. 10 The Augustan Empire 44B.C.-A.D.70. Cambridge: Cambridge University Press, pp. 781-807.
Wells, C., (1992) The Roman Empire. 2nd edn. London: Fontana Press.