Depends, if they used cheap, heavy spokes at a pound a pop, that would leave them £18 to strip an existing, and build a new wheel.
My mate builds wheels for me now & I’d be embarrassed to only give him £18 quid to do that, and that doesn’t include any shop overheads.
You’ve removed a revenue stream from them by supplying the rim as well, so not exactly a great money spinner of a job for them. I think £100 is a touch high, assuming a middle of the road spoke being used, but not surpising, given a lack of opportunity to supply a rim, etc.
You could always teach yourself the first step by lacing it up youself & then getting them to tension it…