With all the frenzy over the Internet, entrepreneurs have been able to create and sell companies for huge sums without ever earning a dime. But in what may be a record of financial weightlessness, America Online said yesterday that it would pay at least $287 million for a company that has never taken in a penny in revenue and has no plans to start charging money. What the company, an Israeli concern called Mirabilis Ltd., does have is users, 12 million of them who spend an average of more than an hour a day on its Internet chat service. Ultimately, America Online hopes to put advertising on the chat service and sell its members products and services. ''When you talk about making money on the Internet, no one is doing more than America Online,'' said Robert W. Pittman, the company's president.