The Villa boss paid Belgian club Genk £8million for the striker's services and the 22-year-old has responded by netting eight times for the Midlanders.
Benteke has been a key figure in Villa's recent surge in form that has seen unbeaten in six matches in all competitions.
The Belgian netted twice at Anfield last weekend and Lambert said: "I think Christian, since he came here, has been excellent.
"He was probably unheard of and nobody really knew too much about him.
"Then all of a sudden he's doing what he's doing and then everybody takes notice.
"He's been an absolute steal. Transfer values are ridiculous but I'm pretty sure he's not worth seven to eight million now! I can't speak highly enough of what he's done."