Be sure you know what the objectives are for the clinical trial in which you are being asked to participate. There are five phases, here, phases 1 through 3 are explained in the NCI video below. The higher the number of the phase, the more value it may be for the patient.
This National Cancer Institute collection of videos will help you understand clinical trial phases, how your safety is protected, issues around paying for trials, and how to find a trial for which you might be eligible.
Although the experimental treatment given in clinical trials is often free, participants are usually required to pay usual care costs. This NCI page discusses what clinical trial costs health insurance plans are required to cover, what conditions must be met for insurance to cover these cost, and what costs plans may not cover. Costs associated for clinical trials.