RM25 is about $8. That's not expensive at all. Is it actually a different price? If its not, change the subject of your post.
It is more expensive than Musang King, so yes it most expensive durian. Off season fruit, or when this or any premium durians reach Singapore prices are higher. That does not really count.
In packaging you see only half durian. Black thorn durian on average is 2 kg, so 50 RM per fruit. That is 17 USD.
Considering average Malaysian earning power about 3 times less than in USA, can you imagine spending $50 on one fruit in USA? For average Malaysian it is very expensive. You can have nice lunch from street hawker for 5 RM.
As far as Nonthaburi durian, it is common Kan Yao (long stalk) durian. You can grafted plants in many markets. In Malaysia this durian still has inferior status. News about $300 durian picked up from original article and than spread like wild fire over the the internet by journalist looking to sensationalize. Probably somebody paid $300 for a durian, I don't know, but I'm yet to find documented proof with any growers, sellers, buyers that these kind of transactions taking place every year. Until further evidence I'm going to put $300 durian in internet gossip category for now.
Also just about every durian orchid in Nonthaburi was destroyed by the floods 2 years ago, so very likely those trees do not even exit anymore.