Reasonable amount to spend on 7 yo party

[LB1291]

I get that it can be relative but for a party for the whole class plus friends?

It will depend what you are doing, what you chose to buy in and what you do yourself, and mainly what you have to pay for a hall.
Some people will get their local Church hall, or Scout hut or whatever at a cheaper rate or even free, because they volunteer there. Others will have to pay commercial rate. Some people will do games themselves, others feel the need to pay for "professional" entertainment. Some will put together food for a lot less than others.
If you are doing such a huge party, then I guess that means you aren't paying a 'per head' activity (like at soft play or paintballing).
Some community centres do things like rollerskating discos or something for a fixed price, and not per head.
This questions has so many variables.
Ultimately, you 'cut your cloth' as they say. Decide what you can afford, and work back from there.

Depends what you're wanting to do for the party - whole class church hall and entertainer and you sort a buffet is probably fairly cheap... but I'm finding now that we're starting to move past that sort of thing (mine is currently planning her 7th birthday party with terrifying precision... and her birthday's not till bloody next year). She's adamant she wants a trampoline park party which we've said is only a small group of friends - and she's fine with that, has the guest list all worked out in her mind, knows what cake she wants and everything. I think Reception/Y1 tends to be peak whole class parties and we're now dropping toward smaller group ones (god help my youngest who gets left out of everything then).

How long is a piece of string?

A party at home can be relatively unexpensive but it could also cost thousands.

At that age we usually rented a hall, did party games ourselves, provided Birthday tea, cake and party bags. Usually cost less than £200. If you hire an entertainer you could double that easily.