My DS has had birthday parties every year since his 6th birthday. They usually cost less than £150 but you could probably do them for under £100 if you cut out pinata, character mascot, party bags etc.
If you have a big garden you could get that party gazebo from B&Q. Or do you have a big enough room for them all to sit down in with an entertainer like clown, animal man, magician, mad science etc.
I have had a party for 55 kids and 1 for 30 kids. Both were quite chaotic. For the 55 kid one I had a bouncy castle in a hall (nightmare unless you can get an adult to man the bouncy castle at all times and make sure too many don't get on all at once. We actually had a nose bleed due to having too many on at once). I also had an animal man which was £50 for an hour (Fantastic, however I think 55 kids was too many. They lost interest because they were having to wait too long to get a turn at touching/holding an animal.)
For the 30 kid one we had a dance class party in a dance studio. This was £75 for an hour in the dance studio and an hour in the food area. My DS really enjoyed himself but a lot of the boys wouldn't join in because "dancing is for girls."
This year I have paid £30 for a big hall and £50 for a DJ that does party games, dancing and has a bubble and smoke machine.
Next year we plan to have a multi sports party. The local secondary school hires out it's school gym plus a room for food for £42.50 for 2 hours in the gym and 30 minutes in the food area. They provide the sports equipment so you can either do the games yourself or do what I will do which is hire a sports coach for £50. You can get 8 x medals for 99p in the 99p shop to give out at the end.
Our local leisure centre also does cheap parties, so it's worth looking at yours.
If you shop around you can get a venue around here for £20-£50 for church hall, community hall, school hall, british legion, scouts hut, pub function room etc.
Also ask local dance instructors or zumbatomic teachers if they do parties. I looked into this and if I sorted my own venue they charged £25-£35 per hour.
Music Bugs (you have to arrange your own venue) - for up to 20 children - 30 minutes = £65, 45 minutes = £80 and 60 mins = £95.
However ask around as I found a self-employed lady who wasn't part of a franchise that did sessions very similar to Music Bugs and charges £30 per 30 mins for parties.
Private hire at a soft play near me is £100. It's double that at another local one, so try different ones.
A big hit at both parties was hiring a mascot. We have had Mickey Mouse and Jake and the Neverland Pirates. It was £25 for 30 mins including a gift and the kids loved dancing with them each time.
Another good idea is to do a cooking party at home. Chad Valley cook book or I Can Cook have good meals to choose from. Then they can eat what they've cooked so you get enterainment and food in one!
If you plan to have a pinata (always a hit with kids) make sure they all line up to take their turn and stay in the line. I tried to do this but then all the kids got out of the line and crowded round the pinata. How we didn't have another accident I don't know and I felt it was quite dangerous. Also the kids were all fighting over the prizes so what I would do next time is put in the party bags closed with elastic bands with 1 x punchball balloon, 1 x bottle of bubbles, 1 x bouncy ball, 1 x sheet of stickers and 1 x glowstick so there's one bag each rather than putting everything in in loose and some kids getting loads and others having none. The first year I didn't do a pinata or party bags as there were more children. I decorated the hall with normal balloons but blown up with helium and gave each child one to take home.
For decorations and plates, bowls, etc pound shop, 99p shop, Asda and Wilkos are really good. Don't bother with cups as you always get loads of spillages. Also so you don't get loads of wastage put the food out on the plates ready and give them out. I did:
Pizza, hot dogs, fish fingers, breaded vegetable fingers, chicken nuggets
French fries
Cherry tomatoes/cucumber/carrot sticks/peppers
Strawberries/grapes
Birthday cake for pudding
Cup drinks with straws (12 x 200ml for 79p in Home Bargains)
For pinata/party bags:
Bubbles (9 x tubs for £1 or 10 x bubble wands for £1 in Poundland)
Punch ball balloons - Play.com do 50 x punchball balloons for £4.02 with delivery
Glowsticks - 20 for £1 in Poundland
Stickers
Bouncy balls