Hi,
You could also try Tunisia First. I'm not sure if you can book on line, but they do have a website you can look at before you call, which will give an idea of prices. They also offer various specialist tours and holidays and offer locations that other tour operators generally don't, such as Bizerte and Tabarka. Their prices are probably not always the cheapest, but they are certainly competitive and they do deals of the week which are good value, as well as early booking offers. The other advantage is that you can piece together your holiday exactly as you want.
http://www.tunisiafirst.com
Hope this is of some help.
A.