The analysis of the blood is relatively cheap. However, investigating the DNA of the tumour for mutations in the first place is still expensive.
The price is coming down as the field of cancer medicine moves from treating tumours in whichever part of the body they are discovered, towards drugs that target specific mutations in tumours.
Dr Nick Peel, from Cancer Research UK, said: "Finding less invasive ways of diagnosing and monitoring cancer is really important and blood samples have emerged as one possible way of gathering crucial information about a patient's disease by fishing for fragments of tumour DNA or rogue cancer cells released into their bloodstream.
"But there is some way to go before this could be developed into a test that doctors could use routinely, and doing so is never simple.