I just checked out tarsnap. It's $0.30/gb/month. I currently do a backup to S3 which is a fraction of the price. I wouldn't mind having another cloud service. Why did you choose tarsnap if it's so expensive.
Many of us are using Tarsnap for actual businesses, where it is criminally underpriced. Among the many reasons to use it over backing up to S3 is that Tarsnap backups are encrypted and the service is written by a domain-expert in that field. Among many other possible scenarios, I use Tarsnap to back up the MySQL and Redis dumps for Appointment Reminder. These include some data which is covered by BAAs which bind me to restrictions on privacy and security under the HIPAA legislation. Just scp'ing them into S3 would be an automatic compliance failure, where I'd pay $600k or so of fines to save $2 a month on Tarsnap.
At $0.5/month it's pretty cheap in fact. But while the price per picobyte might be higher than AWS, I feel a lot more comfortable using something that has been purpose built for the task by a legitimate genius. The whole point of backup is to be able to sleep at night, I don't want to cut costs here.