Cost is probably not as much a problem as finding a 5 speed,they are getting scarce. I have forgotten the Renault model they were used in but a little internet research should sort that out. You also need a new crosspiece for the rear frame ehich is probably hard to come by but I would try Sports Car World in Texas. I haven't looked in my manual but seem to recall there are a few other small pieces secessary. I have the four speed in my Special and have never bothered to replace it with the five as the fifth gear is an overdrive and probably of very limited benefit. My car with the four speed still manages 40 mpg on the highway....Dave
I don't know the site name but there is a very large Europa club where you could most likely find what you need. Try Yahoo groups and see if they are there.
Just saw you were here in Texas, with that said, look up Lotus owners of South Texas (LOST) and post your question there directed to John Pels and Brent Powell. They will know how to answer your question.