A trio of researchers at CEP have done a fascinating little study of lobbyist revenue. Their conclusion:
Our main ﬁnding is that lobbyists connected to US Senators suﬀer an average 24% drop in generated revenue when their previous employer leaves the Senate. The decrease in revenue is out of line with pre-existing trends, it is discontinuous around the period in which the connected Senator exits Congress and it persists in the long-term....Measured in terms of median revenues per ex-staﬀer turned lobbyist, this estimate indicates that the exit of a Senator leads to approximately a $177,000 per year fall in revenues for each aﬃliated lobbyist.
So: starting around 2000, the value of ex-congressional staffers started to rise much more quickly than non-connected lobbyists. And when the connected lobbyists' patrons left Congress, either because they retired or lost reelection? Boom. Their revenue drops like a rock. This isn't really surprising, I suppose, but it's interesting to see a number put to it.
And by the way, this is only true for senators. For lobbyists connected to House members leaving office, the loss is "a weakly statistically signiﬁcant 10% of generated revenue." Moral of the story: if you want to cash in, work for a senator.