One of the important variables in the financial model is how many people we plan on putting in the space. Too many people, and its uncomfortable and members won't feel their getting value for money. Too few and the place feels empty and we need to charge too much to make the model work.
There are several discussions on just this topic in the coworking google group. With so many new coworking spaces on the drawing board (almost all in the US) this is something many people need to know.
Here is a search of threads in the group.
You'll see that 100 square feet (9.3 square metres for the all of us in the rest of the world) is what many have found works. At least one is as high as 160 square feet per person, but planning to lower that to 130.
Based on the space I am looking at, which is a rather large 400 quare metres, that implies about 25 to 40 people (at 160 down to 100 square feet each).
My model, at the time of this post, is allowing for about 24 people in the office on a normal day (12 permanents at their desks and 12 casuals at shared tables). This means either (a) the space will be very luxurious, justifying a decent price per person; or (b) we can put more people in.
So another piece falls into place... I'd hate to have found out that we could only fit 20 people in and needed 30 to make it work!