Everyone needs some safe place to store backup of their files. Google Drive, OneDrive (SkyDrive), Dropbox and some other such cloud storage companies are available which provide you storage to put your files in. Most of them offer some free storage e.g. Google Drive, OneDrive and Dropbox offers 15GB, 7GB and 2GB respectively.
If you are getting out of storage, you can buy some more storage by upgrading to any of the plans available by your storage provider.
Which one of three mentioned is low-priced?
At this time Google Drive is the one you wanna go with. They are giving free 15GB and you can get 100GB for only $1.99 while OneDrive and Dropbox are charging more money than Google Drive.
Google Drive pricing
Google drive's storage is shared between Gmail and Google+ photos. Pricing is available on storage settings page.
OneDrive also gives up to 8GB of storage for free if you install their mobile app and refer friends alongwith 7GB.
Dropbox upgrade page
Related: Free cloud storage by Dropbox
All these three are among most popular storage providers (Don't get confused with OneDrive, it is actully SkyDrive.) and Google drive recently reduced prices for different storage plans and I hope others are going to reduce too. Until then Google Drive is the one with lowest price.
If you are running out of storage but don't wanna spend money then you can sign up with multiple companies. Get upto 34GB of free storage from different storage providers.