Our school has not done an assembly, but ourCub scout pack had Mad Science in for a pack meeting last year. It was $250 for the hour long program, about 90 boys and at least one parent per boy attended, plus numerous siblings. It was the best science night we ever had. They kept the kids attention, got them excited about science. The school brings in COSI on Wheels, another science program, which costs about $1500 for 2 full days. (COSI is a science museum---if you have a local science musuem, it is probably worth checking out) If you can afford Mad Science, I'd say do it, they put on quite a show.
That sounds about right. Seems like it was $60 or $70 for the 6 week session. Ours is not a wealthy school (40% economically disadvantaged), so that was a concern of mine the first year.
However, they only needed 15 (I think) for the class to make and they filled to capacity. The next year they ran a couple classes, one for younger and one for higher ages. (We're K-5).
At one time, long ago, the PTO looked at getting involved, maybe contributing and bringing down the cost per child. But since the afterschool program only accommodates a certain number (20?) it wasn't an investment we thought would benefit the school as a whole. So we didn't pursue it - and there were plenty of other ideas on the table at the time.
But just the regular commercial offering seems to have worked well. The classes either make or they don't. The school does nothing but furnish space and send the flyers home in backpacks. And they do get some perks from it.
Our school tried to do the mad science after school program but too few people signed up. I think cost was the issue. It was roughly $10 per l hr session per student.