Microsoft has recently unveiled Outlook.com Premium, a service that (once again) allows you to use a custom domain with outlook.com. The introductory price is $19.95/year, regular $49.99.
This seems to offer very similar functionality to Office 365 Business Essentials which also allows for a custom domain; standard pricing is $60/year (or $6/month).
I appears that the most significant difference, beyond the $10/year difference, is that up to five users can have accounts with the custom domain at Outlook.com Premium while each new Office 365 Business Essentials account is an additional $50/year. Of course, each Office 365 account includes 1 TB of OneDrive for Business storage.