I think if Ayu held an a-nation Singapore style concert worldwide it would sell out and be successful. The Marina Bay Sands has a max capacity of 1680, and with Ayu's style of stage probably yield 1400 seats. From reading everyone's reviews online, it seems like the overall reception was extremely positive.
Seats in US and Europe are usually variable price (the closer you are, the more $) so avex can recoup a lot of money from front row seats that I'm sure some fanatics will be more than happy to pay ~$250+
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So realistically she could have a show(s) in Singapore, HK, Taiwan, China, NY, LA, London. Maybe even Seoul and Brazil (large Japanese population).