Well, it's $15/user for unlimited storage. Google Apps is $5/user for 30GB for each user, and then you have to start buying additional storage with "licenses"[1] which brings the price a little closer to Dropbox if you have a lot of data. I only use a Google Apps account so I don't know how well the clients work with multiple accounts (private Gmail and Apps). Maybe Dropbox will handle it better?
Obviously, Google Apps gives you much more than just Google Drive so the comparison will greatly depend on what other services you need/use, what your employees and contractors already know, and whether you have Linux clients since Goolge Drive still doesn't support Linux.
I'd be willing to wager that someone at Google has figured out that 30GB/user covers > N% of users, where N ~ 80-90. This makes Drive more attractive than Dropbox for that N%.
Obviously, Google Apps gives you much more than just Google Drive so the comparison will greatly depend on what other services you need/use, what your employees and contractors already know, and whether you have Linux clients since Goolge Drive still doesn't support Linux.
[1] https://support.google.com/a/answer/1726914