My experience across churches and businesses alike is that very few are willing to pay (usually the same amount as for a retail mailbox per month) to back up Office 365 data. Microsoft rarely if ever loses data from their server side of things as far as I can tell (they do copy data between geographically redundant datacenters, and they retain deleted items for X days (configurable) but they don't "back it up" as in, let's go back to the version of the mailbox from two months ago). We can sell backups through a couple of different providers, with unlimited data storage and retention included (keep in mind, backing up old mail/versions will take more space over time than the live mailbox) and even free storage of old mailboxes that have been removed from Office 365, for about $4 to $5 per month per active mailbox and about the same per SharePoint account. But even customers who have deleted data from their mailboxes (almost always through user error, though they do sometimes claim they didn't do it), when offered backup, always turn it down.
It's REALLY hard for people to justify paying anything reasonable for backup when the mailbox itself is so cheap (or for nonprofits, free) and mostly works very well.
Not that it's not a good idea, but your "I'm reluctant to pay" is not uncommon
Then again, getting people to pay for good backups is always difficult--but the ongoing cost and how comparable it is to the primary service which is already very low seem to conspire to make it harder than even other backup solutions to sell.