You have mentioned backup and not 'sync' so for backup there are some cheaper (and good options) - BackBlaze, CrashPlan, Amazon Glacier (dirt cheap). First two

You have mentioned backup and not 'sync' so for backup there are some cheaper (and good options) - BackBlaze, CrashPlan, Amazon Glacier (dirt cheap). First two are not exactly 25/Yr but are cheaper than SpiderOak especially if you go for longer subscription. Have been using CP for an year now but BB's interface is a lot better. See if you can setup your own OwnCloud on your Linux server(this is an open source Dropbox like service).