You'll need to compare it with weapons like
Reaver which has better resists and a 40/40 ratio but no extra stats. That weapon goes for 10k'sh, so I'd put this weapon in the same ballpark personally. I'm not sure the extra stats push it beyond the 10k mark due to the big ratio difference.
A better comparison might be
Argent Protector which I think is objectively better in all areas and goes for 8-10k. Based on this comparison alone I'd put the Katana in the 5-8k range. Markets are sort of weird on rare items though so it's possible someone would pay more. I personally don't think 20k+ is a reasonable asking price. I'd probably start it at 8k and be fine getting half that.