Our local zoo has a birthday party area, and they arrange it for you. I would love to have a party for DS there.
But it doesn't include admission. So on top of the per person party cost, there's the admission to think about (and since our parties are still adult friends and relatives, that's all adult admissions). I cannot even imagine asking friends and family to pay their way in (if they were all members of the zoo, that would be one thing, but they aren't), and I'm not going to more than double my cost for this party, so no zoo party.
Our zoo parties include admission to the zoo but the parents have to pay to park. It is only $5 but if I paid for all the parents to park, that could easily add another $100 to the cost of the party. On the other hand, I don't want to ask the parents to pay...so no zoo party for us either!
We have NEVER asked a child to pay to attend a party. My son's last birthday was at a laser tag, video game place. We paid for the kids to play laser tag and for $15 game play cards, (We only spent $5 on each card, as it was a special deal with the part package.)
I always try to specify in the invitations what we are planning to do. This way, parents will know if we are serving a meal or just cake. I hate getting an invitation for a party from noon-2:00 and not knowing if I should feed my child lunch. I usually do, just to make sure, but more often than not the parent provides lunch. I just think that it is helpful to all to be specific and not assume.
We have never been asked to pay for our child to attend a party. I think that is just plain weird!