Burtons want 3k for a new one, so I would say a min of £1500 plus for a standard bore used one and less if its close to final bore or needs lining.

Paul L no doubt has a better idea of current prices

One of the owners of a genuine 500 bhp plus original race 500 has just had his block linered and rebuilt ready to race this year by Geoff Page so they can be salvaged.