I put no money into my Saabs and just drive them until they die. Thus most of my repairs are just bodge jobs to keep it on the road. I do have decent supply of parts cars now so I can do a better job, but ultimately I do as little as possible. That being said I am considering a tranny swap on my auto boxed current ride. Why? because I have a couple of decent M-Trans to choose from and clutches and all that. The reason I would do it is the exact same one you're considering...possible 10mpg or more fuel economy improvement. I do 500km a week, so that adds up pretty quick.
What's my point? Find a dead parts car with a reasonable M-tran and do the swap if you're so inclined. But dont bother if you're going to have to shell out $1500 to buy all the stuff and have someone do it. You should be able to get a decent parts car for free or at least less than $100. Plus with the price of scrap metal these days, you can probably sell it back to a scrapper for the $100 quite easiliy.
What's my point? Find a dead parts car with a reasonable M-tran and do the swap if you're so inclined. But dont bother if you're going to have to shell out $1500 to buy all the stuff and have someone do it. You should be able to get a decent parts car for free or at least less than $100. Plus with the price of scrap metal these days, you can probably sell it back to a scrapper for the $100 quite easiliy.