I think that there can be a huge price swing here (from $700 or so for a do it yourself "k" t top to several thousand dollars). As an exmaple, here is a t top from Atlantic Towers for $1,390
http://www.atlantictowers.com/at_sub/products_fr.htm
Here is another example of a k top for under $1,000.
http://www.specialtyalworks.com/prod03.htm
To a certain degree I believe that tee tops are like anything else, in that you get what you pay for.
Good luck