You actually want to try to upload 600GB of data with the (likely) slow upload speed of your Internet connection? For reference, if you have a 640kbps upload speed and don't use the upload side of your connection for anything else, it will take about 2100 hours (just under 90 days) of non-stop uploading to complete the initial backup. I think I did the math right on that..
With that much data, you'd be better off to get a USB/eSATA external hard drive to store the data and keep the drive at the home of a friend or relative - or a bank deposit box if you want to be really secure about it. Not only will this be dramatically cheaper, but also much faster than trying to move that much data UP on a residential Internet connection.
As a side note, "cheap" and "reliable" rarely go together. You either get one or the other, not both.
Anyway, to give you some actual online backup options to consider:
One of my small business clients uses Axis Online as a secondary backup for their most critical data (about 25GB). It seems to work well after the initial backup (that took about four days on their business DSL line) but it's not cheap, in large part due to the emergency recovery services that they provide including multiple older versions of files, and next day shipping of a hard drive containing all of the data to their office if it is ever needed. It costs about $50 per month for 50GB of storage.
These are a few random sites I found by doing a search for "online backup" on Google:
www.idrive.com does offer a service with 150GB of storage for $4.95 per month but I don't know anything about their reliability, security, or recovery options.
www.carbonite.com claims to offer unlimited online storage for a flat rate of $49.95 per year (or $134.95 for three years) but the few reviews I found were very mixed with a few good reviews and a few very bad ones...
mozy.com has a "pro" plan where you determine how much storage you need and provides professional recovery services, but it's pretty expensive with a monthly cost of $3.95 plus $.50 per GB of storage used.
If you want to do some research,
this article from PCMag lists what they think are the best online backup services.