>Money doesn't have value because some have it and others don't. It has value because it represents actual resources.
People price goods according to purchasing power, when you decide what price to sell your labor for, you take into consideration your expenses and how you'd like to live.
These prices, however, are limited by competition, you cannot set your price higher than the competition without that resulting in a loss of business or in a lost job opportunity.
Supply and demand is quite easy to understand, bigger supply + lower demand = lower prices, bigger demand + lower supply = higher prices.
With UBI, you're removing part of the population from the work force, even if you think 1000$ might not be enough for you, people living in small towns might do ok with it, people between jobs will have less pressure to find a job, and people deciding that stressful jobs are not necessary anymore, choosing to do something more vocational (and less productive). This leads to a lower supply of goods.
On the other side, you have a lot of people who now have access to goods they did not before, increasing demand of specific goods (mostly inelastic demand goods such as housing, education and healthcare). This alone increases prices although it doesn't explain the inflation, but...
Inflation is when the money in circulation grows faster than the productivity of an economy, when the demand for money is lower than the demand for products. By removing part of the work force you reduce the productive output of your country and thus increase inflation.
This doesn't account for a growth in the tax rate (VAT) affecting business in a way that give an edge to foreigner companies, boosting imports, increasing debt. The economy quickly spirals into massive deficits and that's when the politicians decide to block imports, creating a black market, and screwing up the economy further.
cont.