This is one of the question that will be central to the upcoming debate. Usually, when we vote on economic issues (like we voted on raise in VAT, more holidays, etc...), we usually have all kind of organisation explaining what their position is and why. So you obviously get the political parties opinions, but also the economic associations (the Swiss Business Federation for big companies, the Union of small-medium business), the employee unions and so on. On this topic, we'll most likely get a lot of opinion from the economics academic world as well.
Also, when you get your voting material, there is a leaflet that explains the position of the government and the parliament (in this case, they will most likely recommend a "no").
Now, the comittee that supports the basic income says that the real cost is 30 billions CHF. The rest of the 200 billions CHF comes from the part that is deduced from the salaries by the companies and from the suppression of a bunch of social programs (we already have a system where the state pays the basic necessities to the people without any other revenue, with some conditions). [1]
They also admit that the "dynamic effect" of the basic income are hard to estimate. This is likely to be the controversial part of the debate. It is such a big shift in paradigm that I think nobody, even the most brilliant economists, can predict what will happen. And it is most likely the reason why it will be refused. It's a big jump in the unknown.
Last but not least, the actual text of the initiative (translated in this comment [2]) doesn't fix the level of the basic income. 2500 CHF is just a proposition and if it is accepted, it's up to the parliament to come up with a law that regulates how this all works. So maybe it could be introduced incrementally.
[1] http://bien.ch/fr/page/initiative-federale#revenudebase
[2] https://news.ycombinator.com/item?id=6499949