Under this scheme there wouldn't be any incentive for a business to raise their prices. Imagine that UBI is implemented and imagine it's effects on a business in a mid-sized city, say a restaurant. The business now has an additional 50,000 potential customers, people who can afford to eat out more often due to UBI.
How should the restaurant owner react? He can raise his prices. But that exposes him to competitors who keep their prices the same. He can lower his prices. In which case he will earn less money per plate, but he now has a pool of 50,000 new customers to draw on, so he can make more money on volume. A good restaurateur would lower his prices proactively and try capture a large chunk of this new, larger market of customers.
The point is it's easy to see how UBI could cause prices to go down, and not just up. It's simplistic and misleading to assert that all businesses will start price gouging in response to a UBI. Populations are more complicated that this, and markets are dynamic.