The speakers cost 99 pounds, and I was quoted 1.5-2 hours fitting time if the dealer was going to fit them, at between 25-45 pounds per hour (depending on shop).
The sound is really much improved, especially the midrange/treble, and as such you don't need to turn the volume up so much with the top down at high speed - the bass is probably a little better than the old speakers, but not by a huge amount.
Recently, the MX-5 owners club magazine had a review from someone who has fitted the next size up Kenwood speakers, the KFC-P306 speakers into their 1.6i MX-5. It appears I was misled by my stereo shop about speaker size, as the 306'es fit exactly into the holes left by the factory-fit speakers, meaning these are a rather better bet than the 305's (too late for me though :-( ...).
One thing to note - both audio shops said that the recent MX-5's (1.8's) with the metal speaker surround have a very good audio system and it's basically not worth trying to improve it - so don't waste your money!