I've just had a quote for a damp course. For the full ground floor (and the price will vary depending on how large the house is) I was quoted �952 for the DPC itself, on top of this you'll have to add in the cost of removing the existing plaster to a height of 3 feet and re-plastering after the DPC. You can reduce the first by removing the plaster yourself but the quotes I had for the full job ranged from �3,000 to �10,000. Apparently (and I'm not sure how true this is) it may cost extra (approx. �50) to obtain a guarantee for the DPC. This is for a fairly standard semi detatched house with 3 reception rooms, a kitchen and a breakfast room on the ground floor - oops, almost forgot - add in approx �150 for the removal of any radiators and a bit more to get them put back afterwards. Most good builders will be able to do the full job for you or call in a series of plumbers, builders and plasterers to work it between them (I found the second option to be suprisingly cheaper!! Work that one out!!)
I'd agree with elvis that you should be able to get the work priced before making an offer but I found that this involved forking out about �95 for a full damp survey - which I thought was a bit of a rip off as I'd just paid �400 to a surveyor who couldn't be a*sed to pull up the corners of a few carpets.