The certificate is paperwork, the policy is the cover
An SR-22 is a form your insurer files with the state confirming you hold the required liability cover. It is not a type of insurance and it does not cost much by itself. What changes is which carriers will write you at all, and what they charge for the underlying policy.
A lapse is reported immediately
If the policy cancels for any reason, the insurer notifies the state, and the consequence is usually a suspended license. That makes paying on time more consequential than usual, and it makes choosing an affordable policy you can actually sustain more important than finding the lowest headline number.
It ends, and the pricing improves before that
Filing requirements run for a set period. Rates frequently improve before the requirement ends, as violations age, so re-shopping annually rather than waiting out the whole term is usually worth real money.