According to Wikipedia, Switzerland had a referendum on 5th June 2016 that also included the "basic income" topic:
By October 2013 more than 130,000 citizens had signed, meaning a referendum on the issue had to be held. Publicity included a truck filled with eight million coins emptying the money in front of the Federal Palace in Bern. Even though the initiative's official text submitted to the vote did not specify any level, the campaigners proposed 2,500 Swiss francs for adults (about 1,650 USD at PPP in 2014) and 625 francs for children per month.
Having or not the right for basic income seems a good subject for a referendum. However, actually proposing an amount seems more like a "technical" issue, as the budgetary impact is hard to be evaluated by laymen.
As mentioned by Wikipedia, the official text did not mention any amounts (maybe due to legal issues), but some amounts were published.
Question: Why did the campaigners of the basic income actually propose some amounts? Aren't these suppose to be computed later, if the initiative is approved, based on budget approvals, estimated impact etc.?