It depends on the artist and their quality and what I get. If you're doing customs depending on the size I'd spend any where from $5 for something small and simple to $15-25 if there's color. Not that I wouldn't spend more if there was the work was really detailed and didn't have a long waiting list.
If it's pre-made art depends on the format. I like buttons, stickers, and book marks I would spend about 3-5 dollars a piece for and could end up spending 20-30 in one sitting. (Curse my pin obsession)
My best advice, Time your self on a completely random drawing, something in between simple and complex. Or get a bunch of little postcard (or smaller) size card and try to stay with in that and time it. If you can do something relatively fast with out costing the quality, it would give you a better idea of what you can do in a given amount of time. That way you can do a time log and scheduled pick ups at different points in the day. I found that when I go to Otakon that these artist impressed me the most, I knew what I was getting and when I would have them.
If you're doing Fan art/pre done art, a large verity and formats are the best. Especially pins. Every one seems to love pins. The more you have to offer the more likely you'll attract some buyers.
I was considering not doing any prints since I don't think my art is really up to par for that kind of thing yet. Drawing is usually the thing I have the most trouble with, could take days to draw something. xD
Instead, I was thinking of just doing custom embroidery hats while there. I'm hoping that with practice I can do them quicker. Currently I can complete a hat in about 3 hours if I use markers in it. I was thinking that maybe I can do names and a little marker design behind it. I could also have a little 3 ring binder of different sample fonts people can choose from. What do you think? What should I charge for this?
I'll also have pre-made hats people could just choose from too. xD Like these three I finished last week: [link] [link] [link]
Na. You get an equal amount in some places. But for the most part original works do sell better in general but at conventions it CAN be the other way around. If your selling stuff on-line ya original can do better. But at conventions ask around from past venders and see what the hot pick is at their conventions. Original or fan art.