Answer: You can offer low monthly fees, whether it is the first month or consecutive months. The minimum fees are dependent on what you can offer through Paypal or through Clickbank. Regarding free trials, you can use a variety of methods. For example, you can create sales page with a free trial subscription which is managed through your autoresponder. You can then offer a “One-Time-Offer” for a paid membership subscription. You can select the content you want viewable by free members and what you want viewable by paying members.
Update 9/23/2008:
FASTmember v2.9+ currently has free memberships. Members can join for free and upgrade afterwards to a premium membership level.