How much would this plumbing repair cost?

My 2 year old son flushed a pull-up (potty training diaper) down the guest bathroom toilet. It's been a week since it happened, the guest bathroom has been flushing but very slowly. We thought it was fine and it would fix itself but tonight we came home from being away for a couple hours and both bathtubs... show more My 2 year old son flushed a pull-up (potty training diaper) down the guest bathroom toilet.
It's been a week since it happened, the guest bathroom has been flushing but very slowly.
We thought it was fine and it would fix itself but tonight we came home from being away for a couple hours and both bathtubs and both toilets in my house were backed up and over flowing with water which caused the master bedroom, the hallway and the guest bathroom to be flooded with water. We will be calling a local plumber as soon as they open but we don't have a very expendable account and I've always heard that plumbers are usually quiet expensive. Im a single mother that lives alone, I have minimal knowledge about plumbing, is there anything I should know to avoid being ripped off?
How much would you estimate a repair like this costing?
P.s my brother in law already tried snaking our sewer lines but it did nothing.
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