Because 80-90% of the amount is collected back in taxes, so the net expense for governament is 10-20%, or 30-60€ billion. Sounds like the amount you already use on welfare. The other 80-90% of the basic income goes to people with decent jobs and their tax rates are adjusted so that they only get about the same amount of money after taxes as they now get.
Currently an unemployed person in Finland can get about 500€/month in benefits and also aid for housing, something like 300€. If you add these together and we get a 500-800€ / month range that is the range proposed for the basic income here. The point of the experiment is to test some different amounts. Obviously, depending on the amount there may or may not be other benefits available and taxation also changes.
Like I said originally: the idea is not to give people more money, but to get rid of the inefficient and demoralising welfare bureaucracy that requires a lot of paperwork and does not allow the unemployed to take short term jobs.