Long time ago I created a thread about markups in the audio world:
https://www.head-fi.org/threads/are-most-headamps-overpriced-cmoys.472214/
Short story is that R&D, prototypes, RMA, etc etc cost a lot and then company also has to make a living.
XE USB clock cost $110 on its own(and prolly more by now), board is manufactured by a canadian contractor so it all adds up, also $/€ exchange rate has gone down the drain lately & they certainly get plenty of import customs duty taxes on top, warehouse rental, high taxes on labor, housing, profit etc etc
Last percents of performance never come cheap, that's true in every hobby. If you can't hear any improvement with USB ENHANCER then good for you, you just saved a bunch of money you can now invest in a better headamp for instance
If you have better options and/or feel being conned, you could just go back to your mobo USB output if you dare and live happily ever after, problem solved.
Or you might as well open your own business and sell the same card for 500€ as you appear highly knowledgeable about economics, I'll gladly try it so please advise.