Online storage can be the ideal solution, even for medium sized businesses.
Some offer encryption and compression, with the ability to only transmit sections of files that have changed (so a database is only uploaded once, and only changes sent up thereafter).
Some of the services are very powerful, and track all files, transmitting them as they change (e.g. Carbonite),others run on a timed basis and trawl a whole series of chosen files to find changes / new ones. You gets what you pays for to some extent, some are industrial strength, providing replicated servers et al. Others (like Jungle Disk, based on Amazon's S3 which also supports encryption) offer very low costs but still very capable software.
I back up over 20GB of business-critical data and photographs, updating my online store hourly, but my costs are far from high at under a tenner a month.
As internet speeds improve and server storage gets cheaper, the arguments for off-site backups, managed by a third party but all data being encrypted are getting more compelling.