No, Multimacs aren't cheap and obviously come with other discussions.
I've done a lot of research on this issue, as there are loads of cars you can't fit 3 seats in the rear of properly including Mondeos and even Audi A8's (when I say properly, I mean without overlapping seats mean they sit skew or obscuring seatbelts). Obviously the Panda is the most extreme end of the spectrum.
I'm not looking to make or sell seats directly, but we have designed, made and crash tested an ISOFIX prototype to test the concept. Basically the seat can be offset from centre and securely locked in place, therefore gaining the space in the centre. The result is that everyone is sitting square on the seat with as much space as possible. It makes a massive difference, with 3 seats viable in nearly all cars. As i say, I am not making or selling these, but I am interested in the experiences and solutions people use at the moment.