We do ours at home but never with more than a dozen children. We always have a theme - have done Harry Potter (years ago, when it first came out), Fairies, Pirates, Puppet Making and show, Lemony Snickett, old fashioned tea party etc etc. I organise it to the second as I'm afraid my nerves couldn't stand them running around, so we have a range of games to fit the theme.
I make the invitations, either handmade or on the computer (in fact, I'd love to do this full time!!!), so these are always pretty cheap although time consuming.
I always make the cake to fit the theme too (this probably costs about £6 once I've got things to decorate it). I buy a couple of "big" prizes for pass the parcel etc, again to fit the theme and never more than £1.50 each and then a range of smaller prizes that go in a lucky dip for the other games. Party bags - never more than a pound each and again, to fit the theme.
They have all been fancy dress parties too, so usually spend a couple of pounds in a charity shop for bits and bobs to cut up to make costumes. Food is either hotdogs and salad, followed by icecream and fruit salad, or traditional party tea. We usually try and make the food fit the theme in some small way too.
I don't think I've ever spent more than £50 on a party.
Can you tell that I love organising parties? If ever I get fed up of teaching, that is what I'd like to do - although dh would have to start earning more first I think!!!!