upfront costs probably 4-6k maybe more it depends on which trans and all that is needed, plus if you want it to hold X amount of power.
the down the road cost is the value is droped as its been modified with a diff transmission than came in it so it will lose value, plus most never want an auto in the shelby unless for medical reasons.
Finally, I wouldnt do it, its not bad. IF you have no medical reason why you cant shift its NOT bad at all. Everyone told me I would HATE a manual... but I loved it more and more every day traffic or not lol... plus its faster, more fun to drive, and has more hp/tq due to it
(faster in stock form vs stock auto etc)