do you mean labour cost, or build with new spokes/ rim etc?
I build my own wheels these days, but last time I asked my local bike shop it was about £25 plus parts - if you want a top chap, who charges a bit more Ryan Build Wheels is good, labours £35 from him I think.
IMO its worth finding someone who knows what they are doing as well built wheels are like a piece of art (I dont really know but go very slowly / carefully and have access to tension meters and stuff which helps)