The Ultimate Magic is the 220th chapter in Tsubasa: Reservoir Chronicle.
Yuko explains the Clones about their choice, since they are creations they can be born again. When Sakura asks the price, Yuko replies that she doesn't need payment, for it was she and Clow who started everything.
When she died, Clow thought of her, wanted her to remain. But this was enough to bring her time to a halt and freeze her in the time of her death. Fei Wong tried to grant this wish.
When Syaoran made his wish to wound back time, the timeline was altered, Nadeshiko, Sakura's mother had died, and Fujitaka became an archeologist instead. Clow travelled to the "altered" Clow Kingdom leaving behind those he loved (prior to Cardcaptor Sakura's events), and took Fujitaka's place as king in order to guide Sakura.
The price for Clone Syaoran was paid by Clow's remaining life and magic, while Clone Sakura's life was left for Yuko to pay. She explains too, that the people's prices left in the shop were merely for the sake of her life. When Clone Sakura asks her what'll happen to Yuko, she answers : "That is my own business".