If you only want to simply present your company and one static page will be enough. A simple HTML page, and a HTML editor to manage it, will do.
If you want more. Dynamic content and multiple pages, maybe some news pages or a blog. I would use a CMS (Content management system).
If you want to sell things online professionally you can use one of the available open source cart systems. Like Zen Cart or Magentocommerce (newcomer, but very promising). Depending on your needs and skills this may take quite some time to set up and customize to your needs though.
For the simple static HTML page free hosting/webspace will do. For a content management system I would not advise to use free webspace, and for the cart system you really need a good host which will cost something. It does not have to expensive though. I think I pay about 500 dollar a year for a virtual dedicated server, but for about a 100 dollar a year (very rough estimation) you must be able to find a good host that offers shared hosting without overcrowding the servers (free and very cheap hosting companies usually put to many domains/user on one server, which is not a good idea for professional websites).
The costs to set things up totally depend on what you want and how much you are able and willing to do yourself. If you want an advanced system nut do a little as possible yourself to set up, customize an configure it, I think you will probably easily spend a couple of thousands to set it up + a monthly/yearly amount for updates and such. If you you a able and prepared to do it all yourself it does not have to cost any money, but heaps of time...