it is very difficult to know exactly how many were built, all the sites associated with them are in Japanese and even after a google translate they are still pretty hard to understand. but it may be as low as 500.
the cabriolet was a conversion done by nissan at the factory. they started with a regular micra front end and cut and shut it with a custom rear end, including half custom doors as well.
the result is the cabriolet is about 5mm longer, meaning normal side skirts don't fit properly.
the cabriolets were only sold in Japan, and so any you see that are outside japan will be imports. there are about 10 in the UK, but i think its more like 6 now.
you can import them but its better to buy them inside the UK. i bought my first cabriolet from ebay for £1000, and it was in great condition and had a lot of optional extras like the arm rest, and tailored cabriolet seats. i managed to blag another cabriolet though this site for £300, but it is in shocking condition and is only really for parts. (it had been used as a farm vehicle!!)
the average sale price when i was looking was £2000, but that is for good ones usually from the dealer. i was lucky with my red one, it exactly what i wanted, close to where i was, and a manual too. but now i think the prices have dropped because it is impossible to get parts. people are forced to sell them because looking after them is difficult and insurance is higher too because they are imports.
nex is the guru 
