The 1997 - 2002 Glass window tops mentioned here:
http://986forum.com/forums/general-discussions/58621-glass-window-retrofit-top-early-986-a.html are running about $500 on eBay. It still has to be installed on the frame which can cost well over $500 more to have professionally done. If you can manage to do it yourself, it's a good option.
In my case, I bought the whole car, swapped tops, then sold the top-donor car. Swapping the tops was the easiest part of the adventure.