My kids are 16 and 13. The 13 year old is having his birthday party right now. He has a sleepover with the same 6 kids or so for years now. I get pizza from Aldi, cheeseballs, and a cake (usually I make the cake - this year I bought one due to being short on time.) Everyone has a great time and total cost is... $75 maybe to feed the kids two meals (dinner and breakfast) and play a movie on Netflix. One year the older one got a party at the bowling alley ($8 per kid) and build-a-bear for the youngest ($10 per kid plus a happy meal). We never did a huge blowout party like the laser tag and all that, although a lot of their friends have these kinds of parties. Honestly, my son loves having the sleepover and has never asked to do anything different. The older one likes to just be taken out for "fancy dinner" with family.
I honestly don't understand spending $300-400+ on a kid's party.
I don't know where you are, but near me, the Build-a-Bear birthday parties cost $35/kid. My daughter went to one a few years ago, which is how I found out the cost (from the mom who threw the party).
We're doing a party at home for our daughter next month when she turns 11, which will be the cheapest of the three kids' parties this year. She only wants 5 girls from school to come, and we'll do a home pool party. Cheap and easy. Probably will cost $100-150 total including food and decorations (including a pinata and candy). Hardest part is cleaning up the whole back yard and patio in advance.
Her younger brother turns 9 right after that, and he wants a nerf war, which we can never do properly at home (we've tried) b/c the other kids end up wanting to go in the pool, and then DS gets sad that no one want to play nerf war with him. Plus we don't have a ton of open space for it. So this year we're going to a local park. The park shelter rental fee (which is necessary to reserve the space) is $205. That's without food or pinata or anything else. So that one will be at least $300, probably closer to $400.
The oldest is turning 13 after that, and he cannot decide what he wants for his birthday party, but he knows that he wants to invite his whole class, and we are actually thrilled about that from a social standpoint for him. I've offered to do an Escape Room party for him (at a place nearby), but he's not sure if the other kids will like it. I do not actually want to host a pool party for 18 kids ages 12-13 b/c we don't really have a lot of space, plus DH installed a zipline over the pool, and I worry about safety when we have more than half a dozen kids here using the zipline at a time, especially at that age (12-13). Smaller kids are easier to control sometimes. But this means that I might be out $400-500 if he takes me up on the escape room place or any other party-type place. Last year we took his closest friends (4 of them) to the movies and then home to our pool and zipline, which they loved. But with 18 of them? Nope. Not happening.