Normally, it is only volunteers hospitalised for phase I trials that are offered payment. This is because they are usually healthy people, confined to a clinic non-stop for days to weeks and can‟t expect any medical benefit from the treatment being researched.
Payment is not usually considered ethical for other types of trials, as it might induce people to take part for money. The information sheet on the trial for volunteers will specify any reimbursement that the ethics committee have approved. This might include re-imbursement for travel expenses getting to and from the clinic, or a snack if you are at the clinic for long visits (of a few hours or more). For very long trials (that are many years long), you might sometimes receive a gift during the trial, that is perceived in some way relevant to the conduct of the trial or health condition you have.
The indirect financial benefit of participating in a trial is the medical care and tests performed specifically for the purposes of the trial will be free to you. Anything that is considered standard care and not being done only for the trial you will usually still need to pay for.