When looking for a betta fish from a pet store, always bring a small flash light with you. This is an invaluable tool when inspecting bettas, not only in helping to determine what color they truly are, but also in helping to spot diseases. I always bring a pen light with me when I go to a store to look at the bettas.
Crowntails are normally priced slightly higher than normal veil tails in my stores around here. My local Petsmart charges 3.89 for a VT, but 7.99 for a Crown. Then again, I would never buy a crowntail from my Petsmart, they don't take very good care of most of their fish. There are many other LFS in my area that sell Crowns for only a dollar more than their VTs (generally 2.99 a VT, 3.99 a Crown), and they are generally in infinitely better health anyhow.
And in most pet stores - the females are cheaper than the males.
The link Virus posted (just scroll up from the comment box on that page) is a good basic guide when looking for bettas. To add onto some of those sections though:
Fins: If the bettas fin edges are dark (like are dark brown to black) and look ragged and/or torn with possible slight curling, the fish may very well have fin rot. This can be cured of course with proper treatment, but this is not a healthy betta. A crowntail is distinct in that it has fin rays which extent beyond the fin webbing, which for some people makes it difficult to distinguish the difference between a fish with fin rot and a normal crowntail. A good way to confirm is to watch the betta and see if it flares (set it next to a cup with another male betta in it if it's not flaring at your presence). When it flares, look at the webbing in between the rays - they should still have smooth, un-torn edges. If they are darkened or ragged, then the crowntail probably has fin rot. Also red, inflamed looking edges on a betta (provided the betta isn't already red in color, then you won't really be able to see this) are generally a pre-cursor stage to the more severe form of fin rot with the darkened edges.
Gills: As they say, flaring is the best time to study these, but also watch as the betta breathes in it's tank. A betta whose gills don't both close evenly against the side of the body is ill. Being even is the key with gills, they should move at the same time, the same distance.
Droppings: What they said in the guide on this is really important! Bettas with white, stringy looking poo are generally ridden with internal parasites, and are unable to properly digest their food. Many of these fish die within a short time. You want a betta whose fecal matter is brownish in color.
Behavior: Other things in this category to look for include bubbles in a betta's cup. If these are present, then it is normally a good sign that the fish is coping well. Fish who persist in blowing bubbles even in tiny store cups are generally very hardy fish, and often a good choice.
A fish who hangs vertically or near vertical (tail up or tail down does not matter) likely has a swim bladder disorder of some sort - this also applies to any betta you observe which does not keep itself in an upright position, and instead leans over to one side. These are not good fish to choose, as sometimes this swim bladder damage is permanent and cannot be fixed. The betta fish should be horizontal when it swims, although you may occasionally observe them taking on a near vertical pose as they reach from the bottom of the cup to the surface for air, they should not remain this way. This is simply a pose you see because the cups are so tiny and the betta really can't move very far anyhow.
Water and other things: Clear water is better. But many pet stores never clean their betta cups out and so they are often filled with fecal matter and are often stained brown or are very murky. This is something you will have to learn to deal with in many fish stores, as they are too incompetant to ever clean the bowls. And some stores have a habit of sing blue-dyed water in their betta cups. This is annoying, and often makes bettas difficult to determine an accurate color on or inspect, hence why a flash light is so important.
Closely inspect a betta for small white spots appearing anywhere on the fish. If you see this, do not get this fish. The white spots are ich, and this is a bad disease (although not incurable). Unfortunately, some stores often just scoop the water for the betta cups out of their other tanks water, water which contains other fish which have ich - this just spreads the disease.
Also look (you'll need a flashlight for this, as you will otherwse rarely be able to get enough light in those cups to properly identify this) to see if the fish has something of a copper or gold 'dusting' across it's scales. This is velvet, another disease. You don't want this fish either.