My design firm charges by the hour. If you want a logo that we did in an hour, we can do that, in an hour. The prices is one times our hourly rate. It is not going to be the best logo, but it will only take us one hour.
If you want to have two planning conference calls, a focus group, then six rounds of comps and five final versions for various mediums, a favicon, a 125x125 banner and more, then it cost as much as time as it takes.
If clients do not like this, then can negotiate the world of crowd-sourceing, getting a cousin to do it and mocking it up themselves.
The problem with all that is that if you want a change later on, you are on your own, If you need a two color version for a silk screen, you might be SOL. If your cheap logo is not 100% vector, good luck putting it on a billboard or wrapping a vehicle.
Just as anyone can work on their own plumbing or get some cheap person to do it, there will always be a market for creative professionals who know what they are doing.
If you are losing work to the design sites, you don't want those clients anyways. Or you can let them get their logo from a contest, then design their business cards, website, make money brokering their ads/print/etc/, and create a long term relationship. That could happen as well.