The success of Jim Carrey's Ace Ventura: Pet Detective caught the industry by surprise, including New Line Cinema. Just weeks before the runaway hit opened, says a production source, the studio balked at paying the In Living Color regular $1 million to appear in its upcoming Dumb and Dumber. A very costly mistake. Since its debut, Ace has grossed more than $52 million and Carrey's price tag has skyrocketed. As a result, New Line had to fork over $7 million to snag the star. The studio refused to comment. Carrey, meanwhile, is mulling over heaps of film offers. ''If the right thing is there for me,'' he says, ''I'll do it. If not, I'm just going to see what comes my way.''