Yes, adapters are totally stupid and overpriced.
However, there is one point that everyone overlooks whenever the old Converter vs. Mod debate comes around: converters can "modify" more than one system.
Mods are great if you have a lonely Duo. Converters start to make sense if you have multiple U.S. consoles.
I have a Turbo Duo and I'll eventually get it modified but I'm not in a hurry--I don't want to have all of my eggs in one basket in case something goes wrong that I can't repair. I use my Mirai adapter with my Duo for Arcade Card games and the Tenonkoe Bank. To make the Duo's card slot last as long as possible, I play all of my actual hucard games through a TurboGrafx with a TurboBooster Plus, using the same Mirai for the imports. The adapter sees more action when I want to play PCE games on my TurboExpress. All of this makes it worth the $.
$70 is as cheap as non-Barney converters get these days and, if your set up is similar to mine, it might be worth it. I picked one up from Telegames a few months ago after five years of gaming made my old one rather touchy. The Mirai converters are good but they do wear out over time. Oops...Having said that, I once again concede to the superiority of system modifications!
(Unless someone can find a Kisado for less than $70)