Well, first of all, I don't think it would cost you $400. At least it shouldn't. I was able to get one installed for between $200 and $300, although I don't remember the exact cost.
I look at it as insurance. It will definitely keep your transmission fluid cooler than it would be otherwise, and it is heat that kills transmissions. So you'll be investing in protection, and it will be effective and helpful. No question about it. Do you need it? Maybe; maybe not. But why not err on the side of protection?
When I was faced with the same question I wanted to get a read on just exactly how hot my transmission fluid was while towing. So I bought a ScanGauge II in order to get that information. I found that my transmission fluid was approaching 250 degrees on some hills. I definitely knew then that I wanted to spring for the auxilliary cooler. After that my temps never went above 230 - even on some very long grades in WV. No doubt a worthwhile purchase in my case. Your mileage may vary, of course.
(BTW - I no longer need my ScanGauge II, and I have it for sale on Craigslist for $85. New they are $160. You probably don't want to buy it, since you are concerned about investing in an older vehicle, but there it is anyway. https://rochester.craigslist.org/pts/6022248237.html)
-Speak