We don't have a budget, just look for things we know the child will like as they're all so different....and whatever we can afford on any given week.
Our kids love to give so we feel it's a worthy thing to spend money on their friends 🌟. I would never limit the amount of parties they could go to either. DD1 and DS2 have to decline a lot due to dance and soccer commitments so parties are their down/fun time and important to go and celebrate their friends.
Our kids are 5,7,9,11 and 15 and I find the 7 and 9 year olds are getting the most invites at the moment. Also prep age is quite a busy year for parties. The 15 year old is invited to quite a few but we do limit them for obvious reasons (if we don't know the family, parents not there, kids drinking etc) but luckily he isn't interested in those ones anyway 🙏