Depends on where the event is taking place, how many people show up. It is great to throw an event like that, but it costs a decent amount of money. I know the product give aways are sponsors and so forth, but they give away a lot of dvd copies, it costs to send people from the film out, it costs to rent the venue (usually), it costs for the equiptment... yeah...
The TGR premiers ended up costing around $2000 to put on at an actual venue (not just a club event) with a good deal and then TGR had to pay to have their crew and blah blah blah... 200 people at 20 each is less than 2000 profit. But then again, they get the exposure and what not and people get the word out and blah blah blah.
If your interest is giving them money, buy movie. If it is giving them more fans, get 2 friends to pay the $20 with you to go and then that's $40 they wouldn't have had at all.
But again, depends on the venue and blah blah blah.