In the NCAA's Division I, only 3 baseball coaches may be paid from whatever sources conventionally pay a university's coaches. I put it this way because the sources and their relative proportions can vary from school-to-school. For example, it's not unusual for state universities to pay their coaches from a combination of both state-funded and booster-funded sources.
The fourth coach (sometimes referred to as the "volunteer assistant" or "third assistant,") must be paid from other sources. As suggested in the original post, camps usually foot the lion's share of the "volunteer assistant's" compensation. However, at universities that receive sizable contracts from equipment manufacturers, it's not unusual to see the Head Baseball Coach direct a portion of his share of those funds to the "third assistant," as well. In any case, the funds earmarked for the "third assistant" are almost always coming at the sole discretion of the Head Coach.
These assistant coaches also often benefit from advantageous housing arrangements; and they frequently supplement their incomes with appearances as instructors at baseball clinics.
In any event, while no "volunteer assistant" that I know of is going to live in the lap of luxury and the job is to be viewed much more as a "ticket" to be "punched" than a means of living comfortably, the term "volunteer" is a misnomer.