I did look at mailbox.org. It's about double for their lowest monthly plan, which is reasonable.
Where I hope Purelymail will shine is that it scales a lot more naturally (once you hit mailbox.org's 2 GB cap, you upgrade to a plan that's 2.5x more expensive), it enables use cases that'd get you frowny faces from other providers (because we charge appropriately), and there's no surcharge for things that don't actually cost more.
Want a dozen usernames? Sure, whatever. Need a hundred users? Sure. Store 2 TB of mail for some reason? We've got you covered.
The downside, as you point out, is that the pricing gets a bit scary and I need to work on making it feel safer.