The tops range from $600 on the cheap to $2500 on the expensive side. I've only seen a handful for sale including that of this website, bimmerfest and ebay for the past few years. The biggest problem is shipping. Most sellers do not want to ship and are looking for a local sale. These tops seem to be somewhat delicate and the cost for shipping/insurance is outrageous. They don't manufacture the tops or some of the parts for them anymore, they are obsolete. To my understanding all of the European spec z4's were outfitted with the fittings for the top at the factory, but U.S. spec vehicles didn't receive the same treatment (go figure). Retro kit for both lft./rt. side adaptation is @$140 per side and you do the install yourself. The cosmetic trim panel covers that cover the lft/rt top fittings when installed on the vehicle is like gold. Top key to screw(lrg. allen key) the top down to the frame of the car-like gold. I can't understand why the original owner purchasing a 50-60K car wouldn't make the initial investment and get the whole package making the vehicle that much more valuable/versatile. I guess it's personal preference though. Hell if I had 60K to spend on a car, what's another $4500 to have the top and ancillaries. It's all rolled into the monthly nut payment and you have the whole enchilada. There is a French company that manufactures their version of the z4 top as well. Different look. Check it out at this url
http://www.dmoris-styling.com/hardtop-z4---photos.html