I was speaking of tariffs to take things across borders. It would be insane, the seceded states would have higher tariffs than China to take items across.
Why? Because you say so? Just because you can conjure a worst case scenario doesn't mean it has any connection to reality.
It is likely that if one state were to go, more would follow. They would probably form some sort of federation and may simply adopt the US constitution with some modifications to prevent the current mess we find ourselves in (forbid entitlements, place limits on commerce and general welfare clauses and restrict voting).
We have a successful model, it isn't like someone needs to invent fusion reactors to make it work.
The hard part is the secession itself, and that is simply an agreement/treaty to be forged. That too has been successfully done by far less capable nations.