Once you figure out what style of Nike shoes you like, write down the style name and size, and periodically do internet shopping searches for them. If you find them for cheap, buy a few pairs.
One good way to save is to buy the recently discontinued models when the new one comes out - usually it's only a cosmetic change, the structure and fit of the shoe stays very similar. I buy Mizuno wave riders for running, so if that shoe is currently on version 15, I will just do google shopping searches for "mizuno wave rider 14" and "mizuno wave rider 13". Just figure out how nike numbers their shoe models, and then you could do the same type of search with whatever style of Nike shoe fits you well.