Could someone give me a rough estimate of how much a dampproof course would cost for a terraced property of about 5 m x 10 m

depends on how thick the walls are, if say 9" solid walls single drill outside would be approx £550-£600

Cavity walls drill and inject from both sides, full re-plastering after hacking off 1 metre. £1200.

You average terrace to remove plaster 1m above floor level, inject dryzone dampproofing cream, re-plaster with damproofing additives, re-apply skirting boards & provide 30yr insurance backed guarantee. £1400-1500