If a person had to choose between buying an esrog or a lulav, which one should he buy? For example, if he cannot afford to buy both or supplies are extremely limited.
The halachos of the arbah minim require that all four be used for the mitzvah. If any one of the four is missing, then the mitzvah is not fulfilled. The case that you specify means that a person does not have all four species.
Rambam Shofar, Sukkah, vLulav - Chapter Seven (emphasis added)
5 These four species are considered to be one mitzvah, and each one is required for its performance. All of them [together] are called the mitzvah of lulav. One may not diminish them or add to them. If one of the species cannot be found, a similar species may not be substituted for it.
6 ... If a person did not bind them together, but rather took them one by one, he has fulfilled his obligation, provided he possesses all four species. However, if he has only one species or he is lacking one species, he should not take them until he acquires the remaining species.
If a person did not bind them together, but rather took them one by one, he has fulfilled his obligation, provided he possesses all four species....
This law is accepted as halachah (Shulchan Aruch, Orach Chayim 651:12)...
You can go into partnership with someone to buy them, but each one must own all four (alone) at the time he takes them to make the bracha. This is similar to the case in which the shul (or the rav for the shul) buys a set and it is passed around to each member as a matana al menas lehachzir However, at the time that each person takes the set, it belongs to him alone. Thus, it does not matter how much each person contributed to the set or which one of the four he may have bought.
Note: On the first two days of Sukkot, the mitzvah can be fulfilled only using one’s own Four Kinds. As such, if using someone else’s set, it needs to be given as “a present, conditional on its return.” You can then use the set—your set—and then you return it, as a gift, to its original owner.