Ok the cost can range from free to maybe $100 per year.
Here is what you need
1. A good camera. At this price you will be taking the pics yourself. The good thing is that you can take as many as you want and then pick the good ones.
2. Microsoft photostory. If you have recent version of windows you probably already have it. It is easy to use.
3. A website that will allow you to upload the wmv file you created on photostory. If your current website won't do it then you can buy a website tonight from godaddy. I would suggest that you get the 10 page deluxe or unlimited pages. There is a lot more storage and bandwidth. Plus there other things you can do with the extra pages.
Once you have it uploaded go to a page on your website and create a link to the upload. You do this the same way you link to a website. Just specify link to file upload.
Publish the web page and then go to the link. Scroll your mouse over it and right click then copy shortcut. That shortcut is the entire web address for your tour. Paste it anywhere you want to link your tour.
4. You might want to get a domain name to use creating 3rd level domains to "point" to your tours. You may also be able to do this with a domain name that you already have. Some MLS are pretty picky about self promotion so you don't want to use a domain name that promotes you. I create a 3rd level domain that masks and points to the tour.
Update 12-29-08: As pointed out in some of the comments below, the wmv file created may not be compatible with with some computers. Before uploading the wmv file to your website it can be converted to a flash video file. This can be done by using a free software download from VideoSoft free studio. Then upload the flash file to your website and create link to that. :-)