Basically, I usually price like this -
random person I don't like, 150%
random person, 100%
nice person, 80%
person who helped gather materials, and RPed well while doing it 50%
anyone I feel has earned it, free
really, pricing IS up to the seller, and it is difficult to regulate, but if you are selling to just the average person, much less than 100% of the lens price can hurt the game economy.