The 22-year-old has plundered 15 league goals this season since signing for Villa for just £7m.
Several top clubs would love the Belgian forward but Villa boss Lambert is confident they can hang on to their prize asset.
He said: "If you wanted to prise Christian away from here you would want a sizeable cheque.
"If you look at modern day figures then £25m is probably a good start.
"We can't stop people looking at him but he is under contract here."