On my 24" Nimbus II I had to tighten the cranks about 3 times, at about 1 week intervals, to make the creaking go away and it hasn’t come back since.
Yup, small fee. 
Considering they’d need to spend at a minimum an hour on it.
For just trueing it up, it wouldn’t/shouldn’t take them an hour…
But that’s not extortionate.
Just use trial and error and lots of patience. It’s never failed for me - apart from when i ran out of patience!
Mowcius
AT my LBS it cost me $25 and they worked out a flat spot.
hey guys, i found the problem, underinflation of the tyre, i kept forgetting that on a uni all the pressures on one wheel.
cheers