I think it really depends on your goals and objectives. For example, I have 2 pins in my collection with my eye on a third. To get started, I obviously paid cash for what I have. I have pins I really like, and plan on keeping them for a long period of time. Others want to build a modest collection, but like trading games out. When this is the case, I assume they pay cash to get the collection started, maybe a 1 for 2 trade with a nicer pin to get two not as expensive ones. However, once a collection is established I know that many trade out pins for equivalent others with little to no cash involved.
I'm sure it also helps to get acquainted with your local pinball community so that guys can trade out games more regularly with other local owners.