I do hall photos almost exclusively. It is usually fun and I can meet a lot of people.
- The main reason I do or don't ask for a photo is the situation. If they are rushing past me or are situated where I can't easily ask, I just skip it. Also, sometimes when they give me a strange look when we make eye contact I also skip it.
- Sometimes I can't tell if they are in cosplay or not. Wearing a closet cosplay or a character that is basically in street clothing is one I would probably skip.
As for charging for my work, I pay to use Zenfolio as a site host that allows for selling of prints and high-resolution downloads. If they don't like the photo enough to reciprocate by buying one of those, they they can just download the smaller watermarked photo for free.