I assumed you were using free DropBox storage and not paying. I have no idea how they compare for paid accounts, but I really have zero need for paid storage. I have around 2500 books, maybe 1/3 of those doubled up with both epub and mobi formats, and I only use half of the initial 6.1GB free space.
DropBox gives you 2GB free. SugarSync gives you 6.1GB free to start with the ability to increase your FREE storage up to a max of 32GB by having friends sign up for FREE accounts.
No payments at all, totally free.