Its a tricky one, we get asked all the time, we would charge £125 including cables, which I think is not unreasonable..
however its always tricky when someone brings in a half built bike as I would prefer to do the whole job and know it’s right rather than find a bottom bracket fitted incorrectly or the tension on a crank bolt too tight, or headsets fits wrong.
so we then struggle to then charge someone with half a build to charge more to correct the mistakes.
but were happy to fit parts bought elsewhere as we cant compete on price alone with the rest of the world, ebay etc.
We dont get the prices from our suppliers to compete.