I'm in the same boat at the moment. The price depends a lot on what type pf roof you want to fit and if your prepared or know someone that can do it for you. There are a couple of lads on here who have fitted them for other people - approx 6 hour job.
I am looking for a navy mohair hood.
www.mx5parts.co.uk currently have genuine Mazda hoods on special offer. Appox €750.00 delivered - then I have to fit it.
Art on webcars is charging approx. €1300.00 fitted.
Another site worth looking at is
www.mazda-mx5-shop.com - there hood is approx. €520 delivered (not a mazda hood though)
A vinyl hood (normal) would be cheaper than all of the above.
I believe its a good idea to replace the tension cables at the same time. Genuine Mazda hoods are suppossed to be good for up to 10 years if looked after - which means mine is at the end of its life.
Hope that helps.
Robin