People always say that the dinner package doesn't cost any extra. Well, it doesn't, assuming that you were planning on eating at one of those restaurants anyways. However, we're staying offsite, have 3 adult (by Disney's pricing standards) children, and don't really have a strong desire to eat in any of those 3 restaurants (especially for the price). If we grab Subway on our way into the park, it'll be under $20. If we eat at H&V (the cheapest), with tax and tip it will easily be $100 more than than $20. So in our case, yes, it does cost quite a bit.
I've also heard that you still have to be there 30-45 minutes early even with those seats. If I have to be there 60-75 minutes early normally, I'm saving what? A half hour? We can wait a half hour to save $100. Plus bear in mind that if the show is cancelled for weather or other reasons, you're out of luck. Cancellation policy is 48 hours for the dinner package. So in that case we'd end up going, spending the $120, and STILL not getting to see Fantasmic. I don't think it's worth the risk (for us)!