I think I pay (here in Aus) 95 bucks for 30GB of data, which has a 1 year expiry.
A month out, I turn on a specific firewall rule on OPNsense to force my Usenet traffic over it. I usually eat up the balance in a day or two, at which point I disable the rule again, and top up the data for another year.
$95 for a year of 4G backup capability ain’t bad. What I haven’t done yet is setup my OPNsense rules so that the heavy traffic doesn’t route over 4G in the event of an outage. I really only want it so I can browse the internet, access email, etc.