From what I have managed to glean cheque and money order were popular, but some services also accepted AMEX and VISA after a time (Toth, 2011, Section 63). Though Mostellor (1999) seems to suggest that services like CompuServe would tack on the hourly fee as part of the user’s subscription in some way, though I could be misreading.
Adjusted for inflation those prices are… ridiculous. Gemstone III during peak hours would be ~$46USD/hr and British Legends(MUD1) would be ~$33USD/hr. I love you, Faraday, but I’m not paying you over five hundred bucks a month for a casual twenty hours of pretendy funtimes.
References
Mostellor, T. (1999, May 16). Genie and CompuServe offer die-hard adventure fans wide new on-line worlds. Richard A. Bartle. Mud.Co.Uk. https://mud.co.uk/richard/pcmjun90.htm
Toth, V. T. (2011, December 31). News archive 1997-2011. Mud2.Com. https://www.mud2.com/CMS/about/news/13-news-archive-1997-2011