Assuming that you're on an ADSL connection at work you could have it back to a NAS at home. The limiting factor would be the upload rate of your ADSL line at work. It doesn't matter what service you went with it would take ages to upload that amount of data. Whereas if you were to use a NAS at home you could perform an initial local rsync of your server at work to the NAS then take it home.
The only cost this way is the price of the NAS and hard disk(s), you may also like to upgrade your broadband connection to an unlimited one. It would depend how often the data changes though.
As for an Offsite Backup service provider, rsync.net
seem to be one of the cheapest I came across. It also supports quite a few different methods for getting at your data should you need to.
EDIT: If you live close to where you work then you may want to consider getting an EPS8 circuit. It's a 2-pair circuit of copper wire that connects two locations directly together and it's quite cheap. You rent the line from BT and can run what ever you like either end. For example Cisco 878s to get up to 4Mbps symmetric bandwidth or a G.SHDSL
network extender either end or even this G.SHDSL LAN Extender Bridge
for 11.4Mbps on a single EPS8 circuit.