What does switching supplier actually cost?
Switching costs are mostly not on either invoice. They are the gap in the middle and the history that does not come with you.
What are the real costs of switching?
The overlap where you pay both, the month where less gets done, and anything that does not transfer: ad history, review responses, integrations somebody set up and nobody documented.
How do I make switching cheaper?
Own the accounts before you need them, get an export in writing, and agree an end date rather than stopping abruptly. Most of the pain is sequencing rather than money.
When is it worth switching?
When you cannot get a straight answer about what was done last month. That one is decisive, and it is usually clearer than any comparison of prices.