Encrypted partial backup in Dropbox
I am not so much familiar with Dropbox, but I'd assume an encrypted sparsebundle image would be suited quite well fro your task. For instance this site explains the procedure. 

In essence you have to: 


Open Disk Utility
Click File > New > Blank Disk Image...

Customize your encrypted image file. I'd suggest to use sparsebundle disk image since it consists of smaller files and will probably result in better syncing performance with Dropbox.
Choose a strong password (and optionally save it in your keychain)
Now you can double-click the image file and it will open as a volume.


This is very safe and efficient. I even use a sparsebundle image for my TimeMachine backups.