I haven't organized conferences, but I have organized PG seminars for my dept. for a year, and I was the only person in charge. I am not sure that you can compare the two things. However, it was a lot of work: call for papers, selecting the speakers, finding right location/ equipment for everyone; dealing with the unexpected (e.g. the speaker withdraws at the last minute!), prepare the posters for each seminar, etc. It was also great experience, and I got in touch with a lot of people who I wouldn't have had the chance to meet otherwise. But of course, I didn't have to deal with catering, organizing accommodation, etc. which take a lot of time on the top of everything else. Moreover, does your dept. sponsor/fund this conference? Because you always need some budget, however small, to organize these things. I hope that these few lines may be of help.