Betahaus has a cafe, which is open to everyone, just like any other cafe. It's a great place to go, hang out, and just meet random people. At the top of the building, they also have a big room with a bar that I have been once at an event.
According to their rates, as of July 2013, you can book a DAYTICKET for 12 EUR. It looks, however, that you also would have to book a BASIC MEMBERSHIP, which costs 10 EUR per month. Plus, you can book five hours (really that little?) of meeting room for 25 EUR per month. Maybe it's possible that only one of you books, then invites the others as guests for a meeting. Complicated. I suggest contacting them.
That being said, Betahaus is most likely the biggest coworking space in Berlin, but that doesn't say anything about it's quality. I've been there many times, also to the Open Design City, but I never used their coworking facilities.
Co-Up, another coworking space that I have visited a couple of times, offers a meeting room for 150 EUR per day.
And, of course, there is c-base, a huge hacker space. I assume they would be happy to provide space in case that it is an open event. Other than that, you can visit them at any time, and hang out there, even as a non-member. You only have to pay for drinks. I have been there a couple of times on evenings, and I don't know if they are normally open during the day. Again, contact them.
I forgot: The Bussiness Class.Net. According to Deskwanted, it costs 15 EUR per day. I used the place in 2008 for one month, extending my stay week by week. No membership was needed. The place is targeted at travelers! It's nice and in a good location, though without a separate meeting room. Just ask, perhaps there are not many guests at the moment.
A good overview of coworking spaces you find on Deskwanted, a Berlin enterprise.