Another option is to use existing php scripts to control forum access and charge for subscriptions. I have been researching this heavily for an upcoming website and I plan on using aMember Pro.
It is a robust subscription/membership type program with support for many payment processors. The program runs $99 and Alex will install it for free on your server within 24 hours (if requested). To control Threads access you need the UBBT plug-in @ $40.
You can read more about aMember here:
http://www.cgi-central.net/scripts/amember Happy Threading...
