Being in a sorority was a great experience, and I would definitely do it all over again. However, I would definitely say that rush/pledging and then being active took up
a lot of free time, so you will need to budget your time accordingly. Depending on how your school schedules fall/spring Rush, you may have events as often as 4 times a week. Once you pledge, you can expect to be busy at least 3 nights a week for the entire year - and oftentimes even more than that. Some events are purely social and thus "optional" (parties, dances, IM sports), but others are not optional and thus more demanding of your time - pledge meetings (these can take place at ANY TIME of the day or night - be forewarned
), sisterhood activities, alumni events, philathropies, chapter meetings and workshops, rush meetings, etc.
Once you are active, you have the option of slowing down your schedule a bit (no more pledge committments, but still chapter meetings, committee meetings, and social events). However, if you decide to take on a leadership position, your time commitments increase substantially. Also, during Rush, as an active member, you will be extremely busy with planning and preparation, carrying out actual rush events, and voting on new pledges. The busiest people in our house were usually the President, Rush Chairs, NewMOC (right after rush was her busiest time), and Social Chairs (they were busy year-round). There are other ways to get involved that don't require such a large committment, but to "Chair" any committee is definitely going to take up a lot of time.
I think that anybody who is considering pre-med (and thus needs to maintain a high GPA) should take this into account when thinking about Rush. It's totally possible to succeed in school and be active in the Greek community - it just takes a little preparation and good time-management skills.
(fwiw, a pretty large proportion of my classmates were Greek before coming to medical school. At least one of my classmates was the president of her sorority. I don't think it's going to make or break your application, but it probably won't hurt.)
PS: One more thing. The cost is certainly prohibitive. I found it possible to take on a part-time job during most of my undergraduate career - these funds were more than adequate to pay for dues. Once again, it's all about time management - but this might be an avenue for you to explore.