What makes a wheel a wheel is the tension in the spokes. Correct and even tension are key. It's impossible to see the quality when buying online, so you depend on they're reputation. Buying in person, you can gauge even tension by spinning a wheel with your thumbnail pinging the spokes. Pitch is dependent on tension, so you're looking for every spoke to ping the same. If you get highs and lows on the same wheel, it's a dog.

Of course, the quality of the materials counts, but that's easy to check out.

Online sellers sell wheels built by machine, which isn't automatically a bad thing. Machines can build good wheels if the operators set them up to do so. However setting the standards higher slows things down considerably. Time is money, so many high volume sellers, set the standard as low as they feel they can get away with.

As for why there's such a big spread, it's the economics of volume. Production wheel shops buy the hardware directly from the manufacturer in decent volume so things like shipping come to less. OTOH, a bike shop doesn't buy enough to deal with the manufacturer, so there's a wholesaler, who adds his markup. The parts also log extra miles in shipping which adds to the cost. Lastly, a machine operation can produce a wheel in approximately 3-4 man minutes. Compare this with a shop, where a decent builder can average 2-3 wheels per hour (that's if he's uninterupted and good).

Compare to building a car on an assembly line vs. one up in a garage.

BTW- there are a large number of good online wheel sellers, so that's still an option in the future. But you have to be selective.