I was wondering what people use as a failover ISP? Are there services that don't charge until you need it? Or don't charge much?

Local ISPs around here generally charge $70+/mo for service. Our current service goes out may a couple times a year for 3-4 hours. Obviously I don't want to pay an extra $800 a year for full secondary service just to get a few hours of backup each year.

I thought about looking for a cellular hotspot that could run on a pay as you go MVNO with the yearly card, except we'd likely need something with an external antenna since cell coverage is weak inside the building.

What service (and modem setup) are others using for their failover system?