Ask Smithy Xtra Toyota Prado fuel injectors

We recently bought a secondhand 2007 Toyota Prado diesel. It has now done 120,000 km and has developed a knocking sound when cold, but goes away once the car warms up. We are told the fuel injectors need replacing at a price of between $600-$1000 each. The dealer approached Toyota on our behalf but they have refused to assist. My research tells me this is a common fault and is happening with cars with less than 50,000 km. I am also told the problem may re-surface even with new injectors. Any advice would be appreciated.