Sure, but think about this: that little side project now has about 100x the traffic it did when my bill jumped.

That still isn't much traffic (at all) in the grand scheme of things, and Cloudflare's lowest paid tier deals with around 80% of the bandwidth. Still, it's not hard to imagine that bill blowing up to several hundred pounds per month had I chosen not to act. That would translate to several thousand pounds over the course of a year. I don't know very many people for whom such an expenditure, particularly if it's unnecessary and avoidable, would be something they'd regard as insignificant.

Putting it into everyday terms: it could have grown into my second largest outgoing after my mortgage. That doesn't really seem proportionate or sensible, so why wouldn't I look for a better deal?

It might be different if you're a developer in SV, but I'd say for most people, that's not insignificant. That's more expensive than most hobbies, I wouldn't spend £100 on a side project if there's an alternative.