Yes, it definitely has to be recalculated frequently. If it doesn't, it will be about as useful as the US minimum wage after some time.
But as I said, most people wouldn't have significantly more or less money than they do today. At least I carefully calculated those numbers so that most people would have pretty much the same, for Germany in 2019. So I don't really expect prices to go up drastically.
It's not that people suddenly have 1000€ extra. Either their unemployment benefits get replaced by that UBI, or they now have to pay an extra 1000€ in taxes.