Cloud based storage of client documents

My clients scan and email me a lot of source documents and I produce files and reports for them from a cloud based accounting system. I'm looking for a secure solution where I can store all of this information, structured to my own design (e.g. relevant directories) that is accessible to me and my clients through a link on my website. Can anyone recommend something that doesn't cost a fortune ? I'm aware of Dropbox but don't really know if it does what I want.

Thanks.