Well... she for sure has a good amount of money for herself... with 3 albums among the Japan's top10 best selling (Ayu only has 1), her singles sales and her tie-ins (that are also bigger and most likelly more profitable than Ayu's), I would say she most likelly has enought money for not needing to work anymore...
Also, her mother was one of the top Japan singers back in the 70s (still holding the record for the most consecutive weeks at #1 with an album, 20), so, I would say she was never poor
Still, in Japan it's forbiden to annouce people's income... They usually announce who paied the most taxes in order to especulate who made the most money... So, we don't know about Hikki at all^^