Op can you tell us about what kind of party it was? I would not send my child with that much money. That is insane. Now if it was a chuck e cheese thing or a build a bear I could understand the parent saying they allotted x amount of tickets or each child will get a teddy bear and if they want to get anything else they may want to bring extra. But to ask to pay for everything? No way. How about having a party you can afford. Some of the best parties my kids have been to are the ones given at home.
This one event is laser tag. It's an expensive adventure just for your one kid to go to, let alone funding a half a dozen of them. I myself would not plan a party for my child at this venue because I simply cannot afford it. These other parents are planning a party for their child and I am expected to fork over extra cash for it, which I am complying to at the moment, but this is not something I can afford to do on any long-term basis with two kids and their various events. I'm going to have to start saying no if these birthday parties are going to run some $10, $20 or $30 outside of what I'm already spending on a gift and a drive across town. This is new to me that these parties are requiring extra cash and I'm wondering what is going on and if this is the new normal.
It's completely out of order. The parents can chose to pay for the party, or they can chose to limit the size of th party to what they can afford, or they could chose to not make it a party, but instead to make it "We're planning to take our kids to play Laser Tag - if your kid would like to join in, the cost is $20 plus whatever he wants for snacks - we're willing to be the responsible adults for the group" in which case they are simply organising an activity and it has no link with their child's birthday.
I know when my brother was a kid, LaserQuest was the thing which he and all his friends wanted to do. So, he got to go when it was his birthday, as that's what he picked. But it meant he only got to have 2 friends go with him, rather than the dozen or so who could have been invited if he'd chosen instead to have a party at home. Same with me - I remember a birthday 'party' which was a trip to the cinema - I got to take 2 friends, as that was what the budget stretched to.
In this case, I'd be inclined either to decline or (if your son is desperate to go) to let him go but simply with a card,not a gift.
Evil Margo would be inclined to decline by saying "Thank you, but we don't wish to buy a ticket to your LaserTag event"