This could vary immensely. Just the basic domain registration and hosting should be somewhere around $150/year. If you need someone for the technical details, programming, graphic design, etc. than you could be looking at upwards of thousands for initial setup and ongoing maintenance depending on the goal of the site, the details of software involved, quality, etc. A site where users can interact could mean any number of things. Setting up a blog (Wordpress, Serendipty, etc.) or forum (phpBB, vBulletin, etc.) can be relatively simple with many hosting providers and most average users can manage something like this on their own. Or you could be talking about something like a complex social networking type site requiring lots of custom programming.
Have a look at Wordpress, there s lot of documentation (free) to help you, and also many themes and plugins are free.Wordpress.org if you have your own hosting (about $10 month, but gives you a bit more freedom), or Wordpress.com for a freebie !There s no problem with making a cheap yet professional looking website with user interaction using Wordpress, and you have a plenty of real examples on the two links above.
