Unfortunately for DropBox, Google and Microsoft can almost give away storage for free as loss leaders for their other products. Hard drive space is a commodity.
I was a bit late to the online storage game and at $2 per month for 100GB, signing for Google Drive was a no brainer over DropBox.
Getting 50GB for two years with my phone (Moto X) makes Dropbox look pale in comparison. And they still don't have something akin to Google Docs or Office Online. Space is indeed a commodity. Office, Play, Google Apps, Ads, etc. are the money makers.
I was a bit late to the online storage game and at $2 per month for 100GB, signing for Google Drive was a no brainer over DropBox.