I'm not 100% sure where you're getting that figure. It's easy to reduce costs by using something I like to call practical pessimism.
Yes, staying on top of folks using reminders lowers the amount of folks who drop out. You managed to secure most of their email addresses when they RSVP'ed, right? There will still be people who are no-shows — even after a reminder — but you need to expect that.
Now, say we have an estimate of 200 people who are going to show up to BarCamp on a Saturday. They're not all going to be there at the same time. Sure, some folks will stay all day, but quite a few morning people have made plans for that evening. Couple that with the amount of people showing up thru the day and you'll still have a pretty steady number.
Despite roughly 200 people being there over the course of the day, we always tend to estimate that we'll only need to feed 120 of those attendees during meal breaks. We try to make sure that everyone there gets fed, but we also realize that we're not a soup kitchen. If people either miss the meal completely or a few folks are gluttons who go to get seconds when not everyone has had firsts, most people won't fault us for that.
So what do I suggest?
Buy enough snacks for 60-70% of your attendees. Don't go too overboard. I'm not sure when exactly your event is (6 PM? 8 PM?) but if it's later, it's almost always safe to assume that people have grabbed a proper meal before your event. If it's earlier, they run a good chance of getting something afterwards.
When it comes down to it, snacks are not a meal. So while they're nice to have, people aren't hinging on you feeding them until they're full.
You want the alcohol to be plentiful, I'm sure. But you need to take into account that you'll have people not drinking (i.e. "I would, but I'm driving.") on top of the flakes. Enough alcohol for 50-60% of your attendees is probably fine. Err higher (and maybe even add 10%) if you're throwing a mixer, but err lower if you're hanging the event off of a presentation. Nobody wants to get up and be the jerk who gets more wine/beer while a talk/panel/whatever is going on.
Somebody can feel free to punch holes in my numbers — but that's how I'd approach this sort of event.