I think it's the "Dragon Ball Super" effect. The show only started airing in english last year and being a Japanese company, who generally cater to the local market first and then the rest of the world after, I don't think they had anticipated the international demand.
Before Super aired, many figures such as Golden Frieza, etc. were being marked down heavily all of the place. Fast forward to today and you'll be lucky to find one at retail. Black was the same. Nobody was interested when he was announced, because most casual fans hadn't seen him in the anime. Once he appeared in the english anime the whole world went mad.
I expect the same will happen again with Goku UI and Jiren. The hype is here now for the hardcore fanbase, but next year when the millions of casual fans watch it there will be another explosion.
Best to get anything Super related now, or it will be harder to find in the next 12-18 months.