Rackspace (the company behind Mailgun) has some pretty great pricing in general. They also run Jungledisk which is great if you want to offer your clients managed cloud storage.
I know Amazon is pretty unpopular here as of late, but I've had no issues with their S3 service. It's more robust than EBS (which is what failed when Reddit went down), and there are plenty of clients you can use to access it. JungleDisk is a good example, and their Desktop version is well worth the price. You can map a drive to it, and encrypt the entire contents if you're at all concerned about privacy.