Cost - As I have said in my previous posts, I don't like to spend a lot of money on equipment.
However, reels are the heirlooms of fly fishing. Many people believe the rods, especially bamboo rods, are timeless artifacts that fishermen pass down through the generations.
I believe it is the reel that is our legacy. As a result, I look for reels that are simple, well constructed and bullet proof. My USA-made Pfluegers fit this discription and I know they will last several lifetimes with simple oiling and washing. Other reels I have used that fit the description were Sage, Tieton, Hardy and Okuma. All these companies make good reels that simple and strong and will last a lifetime or two with normal care.
However the costs vary considerably.
I don't like spending more than $50 on a reel and I know I can always find a Pflueger for $50 or less on Ebay. For those who need to have new reels, the Okuma Sierra (which is a nice simple reel with a good drag) is the only reel available for less than $50.
I have recently tried a graphite reel and noticed that several companies are introducing these "beginnger or back up" reels. Originally I snubbed my nose at the "plastic reels" but recently have started taking a closer look. Of course since they are not made of metal, I am less likely to consider them heirloom reels, but they seem cabable of meeting the frugal test. Although if you want to buy American made, you will not find any new reels in this price range.
Reels in the $100 range are available from Ross, Sage, Okuma, Loop, and any number of companies. These are basic beginner reels and you would not go wrong with almost any of the reputable companies. I would primarily pay close attention to the drag and how well it works.
Once you get above $100, you are out of the frugal zone. Although I was given a Hardy Angel (I actually never used it but I did trade it for a Temple Fork Spey Rod, Tieton Reel and line). The Hardy was a piece of art and something I would never use. The Teiton worked great and handled several salmon while in Alaska. However, I was able to sell it and buy two Pflueger 1498's, extra spools and put some money in my boat account.
Remember also that if you fish for several different species and various conditions, you are liekly to need at least one extra spool and likely another reel altogether.
The bottom line still comes down to budget and need. If you need two reels (one for salmon and another for trout), but can only afford one, you will sacrifice on both ends because of the difference in size and quality of the reels needed to handle these fish.
If you only need one and believe you will get into another type of fishing later on, buy the best you can afford. And if you have to choose between spending money on a reel vs a line, go frugal on the reel and cheap on the line.
Other resources
http://flyfishing.thefuntimesguide.com/2007/12/choosing_a_fly_reel.php
http://www.hooked-on-flies.com/fly_reels.htm