Most VPSes are sold by the package. If you want more disk, you get more CPU/RAM/etc. as well. Sometimes that’s what you want, but sometimes you might just want a big chunk of disk to use for backups or bulk storage.
BuyVM sells storage VPSes at quite reasonable prices. Tons of bandwidth, good CPU for the dollar, and…well, enough memory so you can run typical servers but don’t be looking to VNC to it. I’ve had a storage VPS with them for a long time and can recommend them. They’re KVM-based, so you can do whatever you want. And the AUP supports this – it’s explicitly a storage VPS so if you want to use it for things other than backups, party on.
Vultr doesn’t advertise storage VPSes on their public pages, but if you create an account and login, you’ll find the offer. Good prices and while it’s SSD disk and scaled down RAM, I’ve found their boxes to be performant.
This is a less strong recommendation because we’re into definitely budget provider territory here. time4vps certainly offers great specs for the dollar, and I personally haven’t had any problems with them. The network is not as good as Vultr’s or BuyVM’s, but for my purposes it’s fine…it might even be better for you if you’re in Europe.