Value is a concept used as a part of a decision process. When we choose between two things, we choose the one which has greater value to us. A single thing can be valuable in different amounts to different people. Therefore, value is not an intrinsic property of a thing. If all existence consisted of a single particle, it would not make sense to talk about its value.
Just like our consciousness, what we value is determined by our biology and environment. Life developed in such a way, that when we get the things we desire, we feel physical pleasure or mental satisfaction, happiness… let’s call these positive sensations "bliss" in general. On the other hand, when we don’t get the things we desire or get things we don’t want, we suffer.
Bliss and suffering are opposite to each other and can balance each other. Often people take actions that cause them some pain, because they also bring a lot of pleasure. However, to even be able to feel bliss and suffering a certain level of consciousness is needed. A primitive organism with no brain reacts to stimulation only instinctively, like a robot with predefined behavior. It does not feel pleasure and pain like humans do and in many cases it would be contraproductive to do so, like in case of plants, which are regularly eaten alive by animals. It is hard to say how smart an animal has to be to actually be blissful or suffer, but it is safe to say that organisms without a brain are incapable of that.
To talk about the value of life, we must first have a living organism that can value it. We do value our life, which is why we try to keep it going for as long as possible. However, because life has a value to the person living, its value can also be lost, going down to zero or even negative, where it becomes detrimental to live. When we evaluate life, we predict our future based on our present situation. If we are in a hopeless situation with a lot of suffering expected in our future, we may determine such life as not worth living and end it.
Because we have to live first to be able to value our life, there is no difference between a person, who never existed, and a person, who did exist for a brief time to experience some bliss and suffering before ceasing to exist again. It makes no sense to talk about a nonexistent person gaining some value from briefly existing. A nonexistent person is unable to value anything. This does not mean that we should end our life with a reason that it won’t have any value for us after death anyway, but it does mean there is no non-selfish reason to stop someone, who wants to end their life.