All depends on what you are expecting. If you have sourced all original parts etc and taken a lot of trouble you may well not get what you hope to break even. If you don't count your labour costs and have obtained parts carefully at good prices then you may do ok.
I was once offered £20 for a fair nick [but oldish] galaxy by a LBS - I laughed in his face and walked out. Depending on age type and fittings you could go from £150 to £450 - may be a bit more. Remember there are lots of good bikes out there now for not much more cash. Classic rebuilds can be fun but they can be a money pit and as in the case of classic cars and m/cycles the financial rewards can be poor.
As an example I paid £50 for my Dawes and although I fitted new brooks saddle, chain, tyres, cables etc all it really needed were handlebar tape, chain and a good clean/lube.
Anyhow Good luck.
