That's not a reason to not even try to offer anything though.
You could start with a trial of these 120 persons (select from all classes of society), give them UBI and double their taxes to make it realistic, because the UBI has to be paid somehow. If plenty of them would have less money during this trial and still find UBI worth it, that would be a strong signal.
Few people would agree to be part of that study though, and one might argue that those that would (i.e. by applying) won't be representative of society at large.
Testing only the spending side of UBI is fun but useless, especially with that setting.
1. I'd image the poster would have the same objections. It's not large enough, etc.
2. Why double their taxes at all? You could just give them less UBI. I don't see how this makes any sense. What's point of having UBI if you have less money than you started out with. You've just invented a useless tax, which we already have in spades.
It'd only be less money for some, not for all. Since UBI is largely a redistribution scheme, you'll have to test what happens to those who are supposed to fund it as well. Are they happy? Will they move out of your jurisdiction? Do they stop working as much and rely on UBI instead?
These are important questions we need answered before we roll out UBI at large. If it turns out that everybody who's not profiting from UBI says their good byes and leaves for other countries, you'll have nothing to fund the UBI, and likely nobody to produce any goods either.
What'd be ideal is if you could randomly select a group of people and force them into the system, like it or not. Then observe their behavioral changes. This is likely of dubious legality.
A second option would be to offer people upfront an amount of money equal to the extra taxes you expect them to have to pay. E.g., if someone pays $50k/year in taxes now and would pay $100k/year in taxes under a UBI, you offer them $150k in tax-exempt income immediately to join a three year program, and afterwards they can do anything they like, including changing labor choices such that they pay less than $100k/year in taxes. This would get the microeconomic incentives fairly similar to those under a real UBI.
The issue with that is that I suspect higher-level governments (state, federal) would decide that
a. An individual county imposing taxes that high is illegal b. UBI benefits must be paid out immediately to all residents of the city upon moving there (and now every poor person everywhere wants to move to your city).
Still, maybe there's a clever way to try it out in a fairly self-contained system?
I don't know whether there are upper limits on local taxes. One of Germany's local funding is business tax ("Gewerbesteuer"), which communities regulate by setting their multiplier, the lower limit is 200% (of the taxation base), with most between 350% and 450%. The highest I know of is 900%.
Property tax also varies between 200% (minimum) and 1800% (according to Wikipedia), so there's some room as well.
Combined with an initial donation, that may be possibl. If it's somewhere where people don't naturally move to (e.g. not a major population center), the moving-effect might be very welcome as well, repopulating the area.
Ok, so we should never try anything related to macroeconomics? Let's also never change the tax rate ever again.
There is no proof that would actually appease you. There is no way you could measure the effect on inflation without having an entire currency on UBI. There is no way we could be sure about the macroeconomic effects of UBI without first implementing it.