GoG.com currently has all four of XSEED’s Ys titles on sale at 70% off.
Ys VI: the Ark of Napishtim is 5.99. (Normally $19.99.)
Ys I & II Chronicles is $4.49. (Normally $14.99.)
Ys Origin is $5.99. (Normally $19.99.)
Ys: the Oath in Felghana is $4.49. (Normally $14.99.)
You can also buy a bundle pack (shown on the sidebar of each title) for a total of $20.96.
As a note: the pricing I’m using reflects the US pricing. You may see different costs in different countries.