At some point, you’ll have to ask yourself “How much is my time worth?” The answer is a fair amount, actually. Start by thinking about what you expect to make in an hour as a bartender. The session isn’t likely to be any longer than an hour– and might be quite less– but also factor in the time you’ll spend preparing (shopping, practicing, etc.).
Some people will charge based on the number of people attending, e.g. $30/screen. However, a flat rate can make everyone feel a little more secure in how much is being paid. If two people drop out at the last minute, you don’t want your fee to be a topic of conversation all over again – you’re supposed to be at the fun part!
Also consider the size of the business you’d be working for and how much they might be able to afford. A smaller, locally-based company may not have the same financial resources for this kind of event as, say, the local office of a large tech company. For your first outing, asking $300 is a pretty decent place to start, knowing you can go up later as business increases.
What’s also important, though (especially early on), is letting the potential client know that you’re flexible on price. Some verbiage along the lines of, “For an event like this, I typically charge $XYZ, but I also want to make sure that it fits in your budget as well. I’m sure we can find a price that works for both of us,” can go a long way to putting a potential client at ease.