Elance - I had an associate have a complete e-commerce website built for around $3,000 by programmers from Elance.  I was amazed at the functionality and efficiency of the site - especially for the price.  Elance attracts a broad array of talent from around the world.  Each person bidding on your job will have references, and examples of work they have done in the past.  Definitely worth checking out for a larger endeavor.

Scriptlance - Similar to Elance.  Programmers bid on your project.  You will typically find their rates to be a bit cheaper than Elance, but their depth of knowledge may not be as great either.  You can definitely look to Scriptlance to build basic e-commerce websites. If you have a grander scheme in mind, be sure to check with Elance as well.

Rentacoder - And a final programmer clearing-house.  I do not have personal experience using this organization.  Just the same, you will have an opportunity to review a portfolio of the programmer's work before you award the job to him/her.

I recommend that you check out Site Build It before you start talking to any of these companies.  If your goal is to create a website to sell your product or service online, then you need look no further.  Site Build It is a one-stop-shop for registering your domain name, creating your website content, setting up you web hosting, moving your content to your web host, and getting your site seen by people on search engines.  This tool is hard to beat!.

If your programming requirements are more extensive than this, the resources described above should be considered.  It is very unlikely you can get Custom Website Development any cheaper than you can from the companies listed above.  But like I said - if you are looking to simply put together a site that sells without dealing with the expense of custom programming and miring around in the hard-core techie world, Site Build It is the way to go.