I went on my first tour back in May. It really was not that much more expensive than it would have been to drive out there myself and chase for a week. And this way, someone else got to do the driving and I could read or snooze if things got boring. I guess it would be a big difference in price if one lives in Tornado Alley and can chase from home. For others too, it could be an "ego" thing - wanting to find their own storms. But for someone who lives half way across the country away, and is going to have to pay to fly, rent a car, stay at motels, etc. it was well worth it to me. Ended up seeing several tornadoes in SD and funnel clouds in CO, plus hail, a good squall line, and of course plenty of lightning.
That's not to say I won't ever chase by myself. I lived in Indiana for a few years and did chase a bit on my own then when something was close.
I would definitely recommend a tour if your main object is to see severe weather, and particularly if you don't have a lot of chasing experience. Be sure to check out the reputation of the tour company; I believe Tim has guidelines on this site somewhere (and I know he has them in one of his books). Reputable tours do book up pretty early - I made my reservations and paid a deposit back in January to get my choice of May tours.
Frank Kienast