Hmm.. vvell if someone posts 10 times they get 100 feathers on average and you get 200 vvhen you sign up and not everyone spends all that vvhen they get their first avatar i'd say you could up the prices to be betvveen 100-400 [400 being fullbody and group being the addon prices like you said already] and you'd probably still get a vvhole lot of people.
Either one if fine Which ever you think would look good. Feathers are pretty easy to earn around here! I'd suggest you look around at other's art shops and things then compare them to your prices, and adjust as you feel the need!
^^I agree with maro. Pretty much everyone has stockpiled feathers to the roof.