You can get a one bedroom in Austin for less than $800/month, to say nothing of arrangements that involve roommates or shared facilities or couples who would then have a second UBI.
> I would imagine an UBI will be mostly used by people to exclude themselves from active workforce to "retire" in low-cost destinations. Because that will be the only thing it will really allow doing.
You're forgetting the possibility for supplemental income. Combine the UBI with a part time job and you could live just outside of New York City and spend the other half of your time on the business for as long as it takes until it makes enough for you to quit the part time job.
There is also a whole lot of work that really doesn't care about location at all. You can do software development in Kansas if you want to, as well as anything else that involves selling your product over the internet.
> Say you impose a super huge 30% income tax (anything above that will cause a revolt in the middle classes, and mass exodus in upper classes)
The marginal rate isn't what causes revolts, it's the effective rate. Middle class people don't care about paying $12,000 in tax if they also get a $12,000 UBI. Upper middle class people aren't going to revolt over a difference between the two of an amount similar to what they pay already for the existing welfare system we would no longer need.
And the whole point of using VAT is that billionaires and international corporations can't avoid it.