$800 is crazy. I ran into a guy this weekend at one of those coin operated washes (don't worry I bring my own sponges and brushes) who just did his.
He bought the GAHH top on Ebay and took it to the BMW dealer and was quoted $1000 for the install. After reading the directions he decided to try it himself and said it was straighforward and took him two hours "going slow, since it was the first time". He offered to replace mine if I help saying we should be able to do it in an hour.
I previously replaced a top on a Miata and the fee was $350. It doesn't seem to me that a Porsche top is any more complicated than a Mazda top. Actually come to think of it I would imagine it would be easier to swap out being that a bit more engineering went into the assembly, seems like the Germans are always much more exact about these things. Just look how easy it is to replace brake pads and to swap out Halogens for Litronics.
here's a DIY
http://images.wikia.com/boxster/images/e/eb/DIY_Boxster_Conv_Top_Canvas_R_R.pdf