Just replacing a spoke and bringing it to rough the tension of the rest is a minute job on a front wheel, but takes a bit longeron the rear because the cassette must be removed first. Probably worth $5-10 on a quick basis.
But you usually don't want this kind of work, because it'll have you back with a wheel needing work within a month.
OTOH, properly truing a wheel is a longer slower job, with shop rates from $15.00 -$35.00 depending on the wheel and it's condition. This is the same whether a spoke is replaced or not. So a proper job replacing a spoke will be in this kind of price range, plus the cot of the spoke.
Unfortunately there's no way to know whether you got the $10.00 or $35.00 job. But time will tell.
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