Jadis JS1 MkIII and Zanden 5000 MkIV Signature are both in the $15-20k range new (I believe) but whether you will find any of those used at 1/2 remains an entirely other question. Also, second Accustic Arts Tube-DAC II SE as one that would have to be on your shortlist.
Echoing Racamuti's thoughts in respect of price vs. quality, I would lend Yamamoto's brand new YDA-1B tube dac an ear (appr. $3k in Japan, haven't seen a price in the US or Europe yet). Here's the - sort of "poetic" as usual - description on Yamamoto's website
http://www2.117.ne.jp/~y-s/YDA-01B-new-E.html
Echoing Racamuti's thoughts in respect of price vs. quality, I would lend Yamamoto's brand new YDA-1B tube dac an ear (appr. $3k in Japan, haven't seen a price in the US or Europe yet). Here's the - sort of "poetic" as usual - description on Yamamoto's website
http://www2.117.ne.jp/~y-s/YDA-01B-new-E.html