Consider supply and demand. Say there are 100 people and they all need 1 of resource Z and 100 of resource Z are available. People who have only the UBI can afford the lowest quality of Z but those who have UBI + income will all try to get a higher quality of Z. Net result is that earlier it wasn't possible for low income / no income people to get resource Z, now they can. The rich however will have to start paying a slight premium for finer things in life.