I have found that fire eels around here go for about $18+ at 6-7 inches. It definately seems to vary by store, and each eel looks very different. Some will have intense color, while others are more plain. My two little guys I bought last year for $18 a piece, and they are really gorgeous. My big guy, I bought at the same time for $100. He's not the prettiest thing, but my God is he a character and by far my favorite fish (considering I'm at a count somewhere between 100-150 that really says something). Each eel also will have a different personality, but given patients, they get to know you and can be extremely friendly. I usually pick out the eel that is a little bolder in his personality (aka out and about), and all my guys hand feed and are out all the time. So, the cost is really a case by case basis, and sometimes you pay for a personality, not just looks.
This is him in May of last year at 18". We had just gotten him, and he was really skinny. Since then he has grown 6", colored up a little more, and got beefy.
http://www.youtube.com/watch?v=HlVkCX15z3g