Bribing an opponent to drop out of election?

AP/Washington Times:

FBI probes Democratic lawmaker over payments to get challenger to quit campaign

...
The primary opponent and a former aide already have pleaded guilty to concealing the payment. Two political consultants also have been charged in the case.

The warrant, which was executed earlier this month, states that Brady and his primary challenger agreed that the payment “would be disguised” and that the deception would include hiding some of the money under the purchasing of a political poll.
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I get the feeling that $90,000 payment is less than the congressman would have had to spend in a legitimate contest.