I'm part of a large network- affiliated with US Oncology. Our practice does participate in clinical trials- only phase 2 and 3 studies. Majority is breast cancer, but some lung too. It is a significant commitment and not at all a money maker for our practice. We do it because we can offer a service for our patients and it keeps us intellectually fresh.
Having said that, most practices that I know do not participate in research- it detracts them from making more money.