Steve Case, former CEO of AOL takes to Quora:

A lot! I don’t remember the total spending but do recall in the early 1990s our target was to spend 10% of lifetime revenue to get a new subscriber. At that time I believe the average subscriber life was about 25 months and revenue was about $350 so we spent about $35 to acquire subscribrs. As we were able to lower the cost of disks/trial/etc we were able to ramp up marketing. (Plus, we knew Microsoft was coming and it was never going to be easier or cheaper to get market share.) When we went public in 1992 we had less than 200,000 subscribers; a decade later the number was in the 25 million range. That helped drive our market capitalization up from $70 million at the time of the IPO to $150 billion when we decided to combine with Time Warner to accelerate our transition to broadband and diversify our revenue mix.