I see fire eels on wholesaler price lists for around $15 all the time. I paid $13 for my fire eel from a wholesaler, and it was around 10"- maybe a hair over. Typical fish markup is 65-85%, so you're talking somewhere in the range of $25 retail. Personally, I wouldn't pay more than $35 for a 12". $100 is ridiculous.
I've always heard fire eels were way more expensive, and my supervisor told me last year they were all $30+ from wholesalers. However, the laws of supply and demand follow through- if the store is paying less, you should, too.
And finally, Petland is known for being an awful company. I hesitate before doing any business with them, and don't even like buying dry goods there. They buy from awful suppliers, including puppy & kitten mills, and their reptiles & fish are known for being disease ridden all across the country. You're definitely getting ripped off if you spend $100 on that fish.