What is the gross amount paid monthly in Germany? If spent entirely, what amount of that is VAT taxed back to the state?

~1000€/mo, regular VAT is 7% for essentials like groceries, 19% for anything else, with a few exceptions.

The average worker's income tax rate is ~40%, but they'll also have to pay for health insurance (included in welfare at a subsidized rate), pensions etc.

Thanks for the data. To me, that sounds like the end result is within a whisker of $1000/mo after deducting the VAT “rebate” to the German government when that 1000€/mo is consumed almost immediately on a mix of 7% and 19% items.

Sure. The problem is there's little incentive to work. To achieve that, to make it uncomfortable to rely solely on UBI long-term, we'd have to make UBI lower than our current welfare programs. I don't see any proposals for that, much less any support.