Or check the mark-up from a standard sword to a mono-sword - as a percentage, not as a strait number. Take the price for a katana; apply 80% for the wakizashi, then apply the mono-edge mark-up to that.
.. so a wakizashi for 800¥, * 1.9 for the mono-edge (which adds another -1 to the AP), giving you 1520¥ - or a straight 1900¥ for the katana version.
Me, I prefer bringing dikote back into the game, to be the armor-piercing 'adder' to blades (does not stack with monofilament edges). Katanas go for around 9000¥, wakizashi for 8000¥ ... but you get a -4 to your AP.