What's your time worth? If your fixed overhead is $0, you then, are the only variable cost.
I'd say some portfolio building SOP is necessary -- do 2 or 3 templated sites, for free, for some charities (the true kind of charity...one which will be thrilled with any pro bono site). That way, you get a feel for how long it takes, your real costs, what people need vs what they want in the templates, etc. As it's charitable work (and tax deductable...) you should feel free to severe ties if they try to make it overly complicated or step outside the confines of the experiment. Granted, they should be well informed of the situation and why you are doing them this favor. That way, there's no love lost when reality sets in. When all is said and done, I'd have no problem turning to them and asking them if they would have been willing to pay $x for it had you charged.