Fascinating discussion towards the end of this Engineers & Coffee episode at http://engineers.coffee/episodes/2016/4/ about DropBox moving from AWS. They mention a number of very good points including spending money to simply move to their own hosting rather than adding features, using CapEx expenses to make their IPO appear more attractive and several others.
There are some interesting details to consider about letting your cloud costs escalate. Examples of them might include object storage and data movement. Your architecture planning and design should include not only near-term requirements and needs but also take a long-term view. The long-term view would mean estimating the eventual size of your given given some amount of success with bringing in customers. Consider what gigabytes and terabytes of storage might mean in costs and whether or not you can include approaches that will not cause significant costs later if you are successful.