I'll have to think about it, because I couldn't be stuffed looking it all up. Then could be less stuffed thinking about it, so looked it all up.
Cooler with fan; $200
Trans, cooler, etc. connections*; $286
Bypass; $129.
Fan controller; $75
(*Trans to AN x 2 $70, AN to Barb x 4 $110, temp probe inline fitting $23, hose clamps $20, hose $43, misc. $20.)
Total; $690
Plus 7l of LG6, filter, etc. for a service and additional for the larger volume due to lines, cooler, etc.
It's possible to be done cheaper, but mine sits under the tray, well up out of the way and protected. Bypass lets it warm up a little faster, which is important in Mel. and the fan has come on a few times, usually in traffic or on very hot days and/or a very heavy right foot.
If I had the means to see what the trans temp was while driving, I'd probably skip the controller and use a switch, but I'm also a big fan of letting something do jobs like that so I don't forget or miss a sign to intervene. A switch is a couple of $$, and a controller is $100, but the controllers sole job is watch the temp, make it cooler when needed and that's it. And $100 is cheaper than box full of burned fluid and clutches. If I'd taken a little more time to look at things, I would have got one of the cooler/fan units with a built in temp switch and left it at that. A bit simpler, cheaper and cleaner. Maybe the fluid would run hotter, but well within what it's happy to take.
Alternatively, you could run the cooler to the front of the car, skip the whole fan completely and slip in a bypass to make sure it actually gets to temp.