Fishing with eels can get expensive. In my opinion, the best way to keep your costs down (if you'll be fishing alot of eels) is to make or buy the proper equipment to store them. If you browse around here you'll find multiple threads on how to store live eels.
As far as the cost...if you buy them by the eel, you're looking at anywhere between 1.50 and 2.00 per eel. I got to know a couple of baitshop owners and made deals to buy them by the pound (much more economical)$7.99- $8.99 per lb depending onn the cost from their supplier. Then I went to Cabelas...$12.00 per dozen. That was by far the cheapest I have seen in years. Although it was a great price and I stocked up once, it is not economical for me to go their once a week (cost of fuel, time, and the extra $$$$ I'd spend if I went once a week would put me in the poor house).
Last year I averaged 3-5 dozen a week, so buying in bulk and storing them was my best option. Look around at your local shops and see what kind of deal you can make if you plan on fishing them alot. There are alot of options out there as to where to buy eels, but in my opinion your local bait shop will do what they can to keep you coming back week after week!