Certain hosting providers like Mythic Beasts or GoodCloud appear to be 5-10 times more costly, offering less disk space and various limitations.
What accounts for the higher prices of these providers?
A solid calculation. You pay what your data is worth.
Everyone should be able to calculate. You can’t expect 1TB of storage space for a few euros.
Maybe additional backup, Nextcloud Office or Nextcloud Talk HPB.
More costly than which other provider?
I would think it to be more effective to compare two or three offerings on their given specs and price than making general statements 
For what it’s worth: Scale is a factor that hasn’t been mentioned yet.
If you’re a hosting provider for 100 Nextclouds, you can’t offer the affordable pricing a huge company can offer. The big provider can even cross-finance a net loss through other (profitable) products they sell, if they want to draw customers in through a good Nextcloud offer. (Or they can call up a much higher price, because they can use their good reputation/marketing to reach customers either way)
Or, offering a better price, they may just provide fewer features and/or worse service.
And then there are incredibly cheap and small providers. That’s usually someone in their basement trying to get a foot in the door (potentially without a long term plan) 
It depends from hardware quality, software, level of support, you can get 1TB storage, but then have support replay every several days or providers limit amount of users, apps etc.
Just ask them. They want to charge you more money.