Sure, but if we posit that we have a functioning capitalist system right now, then prices are already at a natural low due to the existence of competition. In markets where competition is actually healthy, prices should have already reached an economically viable minimum. If the government shows up and says "k, we're taking an extra 15% of your income to fund this program", then suddenly your net revenues are lower than you can afford for them to be, and you have to make up the difference somewhere. That means either improving the efficiency of production (which basically means firing workers and working your existing workers harder, barring some technological leap) or raising prices.
Competition keeps prices balanced; it doesn't drive them arbitrarily low. Nobody will sell at a price that doesn't allow them to sustain their business.
1. Using public pensions. While unlikely to be supported by the people making the laws (it is their pension), if they have basic income, they don't need a pension.
2. Move all funds currently in other welfare programs into the basic income program. This both moves existing funding m and gets rid of those departments, removing people working for those departments from the payroll. While my statement may be crass, it's a valid solution.
3. Take some of our military funding. Also give some of it to NASA.
None of those solutions decrease net income, except for people losing their jobs (they can find another one?) or their public pension (they're getting the basic income, and the original idea was that once someone served their term, they'd return to their previous profession, and become contributing members of society).
Competition keeps prices balanced; it doesn't drive them arbitrarily low. Nobody will sell at a price that doesn't allow them to sustain their business.