Jelavic has impressed since his £5m move to Everton from Rangers and Bilic believes that the purchase was a bargain for his former club.
Bilic said: "His best years are still to come – he can become one of the most important strikers in the league next season, because he has no limits.
"He's a bargain. I read about £20m-plus transfers and in that respect his price does look like a bargain.
"It's great for Everton and not for the first time, looking at some others they have signed. They know the formula."
Everton face Merseyside rivals Liverpool in the FA Cup semi-final at Wembley on Saturday.