First backup strategy
Even if you have a 10 to 20 Mbps upstream connection, if your entire backup set is in the tens to hundreds of gigabytes you need to start with a strategy. Because none of the hosted services let you create multiple selection sets backed up to the same data set, you may want to follow this method.
First, create a set of critical data under a few gigabytes. Wait until that set is backed up. Next, add additional gigabytes of selections iteratively, sweeping a larger and larger net, only adding additional selections after the initial backup is complete. During that time, incremental backups will keep your critical files up to date, too.
Finally, include all the files you want. By staging backups (and making a local full archive or clone), you can be sure that you aren't still backing up initial files weeks in and lose some important changes.
Pros and cons of online backup
On the plus side, online backups provide offsite storage without the fuss of rotating drives, can be set to constantly update even minor changes, enforces having at least one additional copy of files, and provides peace of mind for a regional natural disaster, which could damage on- and offsite computers and drives.
On the negative side, online backups suffer from moderate to long delays in restores, without personal encryption password, the compromise of company or a government subpoena could reveal private data, are far more expensive over the course of a year than local backups, and a company's sudden demise could render your current and archival backups permanently unreachable.
The cost for these services splits neatly into two piles: flat-rate storage and per-gigabyte (metered) storage. Of those tested, only Jungle Disk and SpiderOak are metered.
Flat-rate services charge a per-computer rate paired with unlimited storage, while metered services allow unlimited computers to have pooled data. iDrive is an exception, it offers 150GB storage for up to five computers for a flat monthly rate.
The flat-rate providers typically charge about £3 per computer on a monthly basis, although several only allow advance payment for a year or more at a time, or offer a discounted yearly option.
CrashPlan offers a $100 (£60) a year rate for an unlimited number of computers storing an unlimited quantity of data. This is less expensive than all its flat-rate competitors for two computers, and vastly cheaper for more than two.
Jungle Disk costs £1 or £2 per month for using its software, paired with passing along cloud storage prices. Jungle Disk's parent company, Rackspace, charges 15 cents (9 pence) per gigabyte of storage per month, as does Amazon S3. Amazon adds additional fees for data transfers in and out, as well as requests. (See Amazon's S3 page for complete details.)
SpiderOak charges a flat rate of $10 (£6) per month, or $100 (£60) per year, per 100GB of storage.