As artifact and art, nihonto are a bargain. That does not mean they are cheap. Still, the level of craftsmanship you can acquire for under $10k is pretty amazing. When you look at the upper end of the spectrum you’re usually in the hundreds of thousands of dollars and not the millions. Compare that to the rest of the art world. It’s the double edge sword, if you’ll forgive the term, of a niche hobby, recognition vs. accessibility.
For the most part an auction is a place to be certain, not take a chance. You must factor your premiums, taxes and any other fees into the final price, and be sure of the item yourself. You must also be familiar with the auction house you’re buying from and their policies. Not all auction houses will stand behind their description.
It comes down to your end goals, are you buying it as art or an investment? Art is personal, if a particular sword or fitting speaks to you, it’s worth what you pay. That doesn’t mean you’ll recoup the price. Markets fluctuate. I have yet to buy a sword at auction, but I have more than a few auction catalogs, the photography can sure be nice.