An online backup service isn't much use if it doesn't make the process of restoring or recovering your data quick and simple. For example, a service should offer search tools for finding particular files in your backup. It's also desirable for a service to be able to replicate an entire folder-tree structure so that it can help you recover from bigger data losses. Keep in mind that if you buy a plan that covers just one computer, you may have to transfer the account to a new PC if you ever switch your main device or if you need to restore data from a damaged computer to a replacement.
CrashPlan is completely free if you're just doing local backups, but even online backups are affordable, with CrashPlan+ accounts starting at $2/mo (per computer) for 10GB of online backup storage, and going up to $4/mo (per computer) for unlimited online backup storage and $9/mo for unlimited online backup storage for a whole household. You can check out their plans here, and try them free for 30 days with a new account.